Add some basic security policies with sensible defaults

This ommmit contains some security hardening measures for the Hugo build runtime.

There are some rarely used features in Hugo that would be good to have disabled by default. One example would be the "external helpers".

For `asciidoctor` and some others we use Go's `os/exec` package to start a new process.

These are a predefined set of binary names, all loaded from `PATH` and with a predefined set of arguments. Still, if you don't use `asciidoctor` in your project, you might as well have it turned off.

You can configure your own in the new `security` configuration section, but the defaults are configured to create a minimal amount of site breakage. And if that do happen, you will get clear instructions in the loa about what to do.

The default configuration is listed below. Note that almost all of these options are regular expression _whitelists_ (a string or a slice); the value `none` will block all.

```toml
[security]
  enableInlineShortcodes = false
  [security.exec]
    allow = ['^dart-sass-embedded$', '^go$', '^npx$', '^postcss$']
    osEnv = ['(?i)^(PATH|PATHEXT|APPDATA|TMP|TEMP|TERM)$']

  [security.funcs]
    getenv = ['^HUGO_']

  [security.http]
    methods = ['(?i)GET|POST']
    urls = ['.*']
```

2 files changed, 135 insertions, 0 deletions

diff --git a/hugolib/testdata/cities.csv b/hugolib/testdata/cities.csv
new file mode 100644
index 000000000..ee6b058b6
--- /dev/null
+++ b/hugolib/testdata/cities.csv
@@ -0,0 +1,130 @@
+"LatD", "LatM", "LatS", "NS", "LonD", "LonM", "LonS", "EW", "City", "State"
+   41,    5,   59, "N",     80,   39,    0, "W", "Youngstown", OH
+   42,   52,   48, "N",     97,   23,   23, "W", "Yankton", SD
+   46,   35,   59, "N",    120,   30,   36, "W", "Yakima", WA
+   42,   16,   12, "N",     71,   48,    0, "W", "Worcester", MA
+   43,   37,   48, "N",     89,   46,   11, "W", "Wisconsin Dells", WI
+   36,    5,   59, "N",     80,   15,    0, "W", "Winston-Salem", NC
+   49,   52,   48, "N",     97,    9,    0, "W", "Winnipeg", MB
+   39,   11,   23, "N",     78,    9,   36, "W", "Winchester", VA
+   34,   14,   24, "N",     77,   55,   11, "W", "Wilmington", NC
+   39,   45,    0, "N",     75,   33,    0, "W", "Wilmington", DE
+   48,    9,    0, "N",    103,   37,   12, "W", "Williston", ND
+   41,   15,    0, "N",     77,    0,    0, "W", "Williamsport", PA
+   37,   40,   48, "N",     82,   16,   47, "W", "Williamson", WV
+   33,   54,    0, "N",     98,   29,   23, "W", "Wichita Falls", TX
+   37,   41,   23, "N",     97,   20,   23, "W", "Wichita", KS
+   40,    4,   11, "N",     80,   43,   12, "W", "Wheeling", WV
+   26,   43,   11, "N",     80,    3,    0, "W", "West Palm Beach", FL
+   47,   25,   11, "N",    120,   19,   11, "W", "Wenatchee", WA
+   41,   25,   11, "N",    122,   23,   23, "W", "Weed", CA
+   31,   13,   11, "N",     82,   20,   59, "W", "Waycross", GA
+   44,   57,   35, "N",     89,   38,   23, "W", "Wausau", WI
+   42,   21,   36, "N",     87,   49,   48, "W", "Waukegan", IL
+   44,   54,    0, "N",     97,    6,   36, "W", "Watertown", SD
+   43,   58,   47, "N",     75,   55,   11, "W", "Watertown", NY
+   42,   30,    0, "N",     92,   20,   23, "W", "Waterloo", IA
+   41,   32,   59, "N",     73,    3,    0, "W", "Waterbury", CT
+   38,   53,   23, "N",     77,    1,   47, "W", "Washington", DC
+   41,   50,   59, "N",     79,    8,   23, "W", "Warren", PA
+   46,    4,   11, "N",    118,   19,   48, "W", "Walla Walla", WA
+   31,   32,   59, "N",     97,    8,   23, "W", "Waco", TX
+   38,   40,   48, "N",     87,   31,   47, "W", "Vincennes", IN
+   28,   48,   35, "N",     97,    0,   36, "W", "Victoria", TX
+   32,   20,   59, "N",     90,   52,   47, "W", "Vicksburg", MS
+   49,   16,   12, "N",    123,    7,   12, "W", "Vancouver", BC
+   46,   55,   11, "N",     98,    0,   36, "W", "Valley City", ND
+   30,   49,   47, "N",     83,   16,   47, "W", "Valdosta", GA
+   43,    6,   36, "N",     75,   13,   48, "W", "Utica", NY
+   39,   54,    0, "N",     79,   43,   48, "W", "Uniontown", PA
+   32,   20,   59, "N",     95,   18,    0, "W", "Tyler", TX
+   42,   33,   36, "N",    114,   28,   12, "W", "Twin Falls", ID
+   33,   12,   35, "N",     87,   34,   11, "W", "Tuscaloosa", AL
+   34,   15,   35, "N",     88,   42,   35, "W", "Tupelo", MS
+   36,    9,   35, "N",     95,   54,   36, "W", "Tulsa", OK
+   32,   13,   12, "N",    110,   58,   12, "W", "Tucson", AZ
+   37,   10,   11, "N",    104,   30,   36, "W", "Trinidad", CO
+   40,   13,   47, "N",     74,   46,   11, "W", "Trenton", NJ
+   44,   45,   35, "N",     85,   37,   47, "W", "Traverse City", MI
+   43,   39,    0, "N",     79,   22,   47, "W", "Toronto", ON
+   39,    2,   59, "N",     95,   40,   11, "W", "Topeka", KS
+   41,   39,    0, "N",     83,   32,   24, "W", "Toledo", OH
+   33,   25,   48, "N",     94,    3,    0, "W", "Texarkana", TX
+   39,   28,   12, "N",     87,   24,   36, "W", "Terre Haute", IN
+   27,   57,    0, "N",     82,   26,   59, "W", "Tampa", FL
+   30,   27,    0, "N",     84,   16,   47, "W", "Tallahassee", FL
+   47,   14,   24, "N",    122,   25,   48, "W", "Tacoma", WA
+   43,    2,   59, "N",     76,    9,    0, "W", "Syracuse", NY
+   32,   35,   59, "N",     82,   20,   23, "W", "Swainsboro", GA
+   33,   55,   11, "N",     80,   20,   59, "W", "Sumter", SC
+   40,   59,   24, "N",     75,   11,   24, "W", "Stroudsburg", PA
+   37,   57,   35, "N",    121,   17,   24, "W", "Stockton", CA
+   44,   31,   12, "N",     89,   34,   11, "W", "Stevens Point", WI
+   40,   21,   36, "N",     80,   37,   12, "W", "Steubenville", OH
+   40,   37,   11, "N",    103,   13,   12, "W", "Sterling", CO
+   38,    9,    0, "N",     79,    4,   11, "W", "Staunton", VA
+   39,   55,   11, "N",     83,   48,   35, "W", "Springfield", OH
+   37,   13,   12, "N",     93,   17,   24, "W", "Springfield", MO
+   42,    5,   59, "N",     72,   35,   23, "W", "Springfield", MA
+   39,   47,   59, "N",     89,   39,    0, "W", "Springfield", IL
+   47,   40,   11, "N",    117,   24,   36, "W", "Spokane", WA
+   41,   40,   48, "N",     86,   15,    0, "W", "South Bend", IN
+   43,   32,   24, "N",     96,   43,   48, "W", "Sioux Falls", SD
+   42,   29,   24, "N",     96,   23,   23, "W", "Sioux City", IA
+   32,   30,   35, "N",     93,   45,    0, "W", "Shreveport", LA
+   33,   38,   23, "N",     96,   36,   36, "W", "Sherman", TX
+   44,   47,   59, "N",    106,   57,   35, "W", "Sheridan", WY
+   35,   13,   47, "N",     96,   40,   48, "W", "Seminole", OK
+   32,   25,   11, "N",     87,    1,   11, "W", "Selma", AL
+   38,   42,   35, "N",     93,   13,   48, "W", "Sedalia", MO
+   47,   35,   59, "N",    122,   19,   48, "W", "Seattle", WA
+   41,   24,   35, "N",     75,   40,   11, "W", "Scranton", PA
+   41,   52,   11, "N",    103,   39,   36, "W", "Scottsbluff", NB
+   42,   49,   11, "N",     73,   56,   59, "W", "Schenectady", NY
+   32,    4,   48, "N",     81,    5,   23, "W", "Savannah", GA
+   46,   29,   24, "N",     84,   20,   59, "W", "Sault Sainte Marie", MI
+   27,   20,   24, "N",     82,   31,   47, "W", "Sarasota", FL
+   38,   26,   23, "N",    122,   43,   12, "W", "Santa Rosa", CA
+   35,   40,   48, "N",    105,   56,   59, "W", "Santa Fe", NM
+   34,   25,   11, "N",    119,   41,   59, "W", "Santa Barbara", CA
+   33,   45,   35, "N",    117,   52,   12, "W", "Santa Ana", CA
+   37,   20,   24, "N",    121,   52,   47, "W", "San Jose", CA
+   37,   46,   47, "N",    122,   25,   11, "W", "San Francisco", CA
+   41,   27,    0, "N",     82,   42,   35, "W", "Sandusky", OH
+   32,   42,   35, "N",    117,    9,    0, "W", "San Diego", CA
+   34,    6,   36, "N",    117,   18,   35, "W", "San Bernardino", CA
+   29,   25,   12, "N",     98,   30,    0, "W", "San Antonio", TX
+   31,   27,   35, "N",    100,   26,   24, "W", "San Angelo", TX
+   40,   45,   35, "N",    111,   52,   47, "W", "Salt Lake City", UT
+   38,   22,   11, "N",     75,   35,   59, "W", "Salisbury", MD
+   36,   40,   11, "N",    121,   39,    0, "W", "Salinas", CA
+   38,   50,   24, "N",     97,   36,   36, "W", "Salina", KS
+   38,   31,   47, "N",    106,    0,    0, "W", "Salida", CO
+   44,   56,   23, "N",    123,    1,   47, "W", "Salem", OR
+   44,   57,    0, "N",     93,    5,   59, "W", "Saint Paul", MN
+   38,   37,   11, "N",     90,   11,   24, "W", "Saint Louis", MO
+   39,   46,   12, "N",     94,   50,   23, "W", "Saint Joseph", MO
+   42,    5,   59, "N",     86,   28,   48, "W", "Saint Joseph", MI
+   44,   25,   11, "N",     72,    1,   11, "W", "Saint Johnsbury", VT
+   45,   34,   11, "N",     94,   10,   11, "W", "Saint Cloud", MN
+   29,   53,   23, "N",     81,   19,   11, "W", "Saint Augustine", FL
+   43,   25,   48, "N",     83,   56,   24, "W", "Saginaw", MI
+   38,   35,   24, "N",    121,   29,   23, "W", "Sacramento", CA
+   43,   36,   36, "N",     72,   58,   12, "W", "Rutland", VT
+   33,   24,    0, "N",    104,   31,   47, "W", "Roswell", NM
+   35,   56,   23, "N",     77,   48,    0, "W", "Rocky Mount", NC
+   41,   35,   24, "N",    109,   13,   48, "W", "Rock Springs", WY
+   42,   16,   12, "N",     89,    5,   59, "W", "Rockford", IL
+   43,    9,   35, "N",     77,   36,   36, "W", "Rochester", NY
+   44,    1,   12, "N",     92,   27,   35, "W", "Rochester", MN
+   37,   16,   12, "N",     79,   56,   24, "W", "Roanoke", VA
+   37,   32,   24, "N",     77,   26,   59, "W", "Richmond", VA
+   39,   49,   48, "N",     84,   53,   23, "W", "Richmond", IN
+   38,   46,   12, "N",    112,    5,   23, "W", "Richfield", UT
+   45,   38,   23, "N",     89,   25,   11, "W", "Rhinelander", WI
+   39,   31,   12, "N",    119,   48,   35, "W", "Reno", NV
+   50,   25,   11, "N",    104,   39,    0, "W", "Regina", SA
+   40,   10,   48, "N",    122,   14,   23, "W", "Red Bluff", CA
+   40,   19,   48, "N",     75,   55,   48, "W", "Reading", PA
+   41,    9,   35, "N",     81,   14,   23, "W", "Ravenna", OH 
+
diff --git a/hugolib/testdata/fruits.json b/hugolib/testdata/fruits.json
new file mode 100644
index 000000000..3bb802a16
--- /dev/null
+++ b/hugolib/testdata/fruits.json
@@ -0,0 +1,5 @@
+{
+    "fruit": "Apple",
+    "size": "Large",
+    "color": "Red"
+}