Add Goldmark as the new default markdown handler

This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.

If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:

```toml
[markup]
defaultMarkdownHandler="blackfriday"
```

Fixes #5963
Fixes #1778
Fixes #6355

1 files changed, 16 insertions, 6 deletions

diff --git a/markup/markup_test.go b/markup/markup_test.go
index c4c1ee032..669c0a446 100644
--- a/markup/markup_test.go
+++ b/markup/markup_test.go
@@ -29,13 +29,23 @@ func TestConverterRegistry(t *testing.T) {
 	r, err := NewConverterProvider(converter.ProviderConfig{Cfg: viper.New()})
 
 	c.Assert(err, qt.IsNil)
+	c.Assert("goldmark", qt.Equals, r.GetMarkupConfig().DefaultMarkdownHandler)
+
+	checkName := func(name string) {
+		p := r.Get(name)
+		c.Assert(p, qt.Not(qt.IsNil))
+		c.Assert(p.Name(), qt.Equals, name)
+	}
 
 	c.Assert(r.Get("foo"), qt.IsNil)
-	c.Assert(r.Get("markdown"), qt.Not(qt.IsNil))
-	c.Assert(r.Get("mmark"), qt.Not(qt.IsNil))
-	c.Assert(r.Get("asciidoc"), qt.Not(qt.IsNil))
-	c.Assert(r.Get("rst"), qt.Not(qt.IsNil))
-	c.Assert(r.Get("pandoc"), qt.Not(qt.IsNil))
-	c.Assert(r.Get("org"), qt.Not(qt.IsNil))
+	c.Assert(r.Get("markdown").Name(), qt.Equals, "goldmark")
+
+	checkName("goldmark")
+	checkName("mmark")
+	checkName("asciidoc")
+	checkName("rst")
+	checkName("pandoc")
+	checkName("org")
+	checkName("blackfriday")
 
 }