Add /config dir support

This commit adds support for a configuration directory (default `config`). The different pieces in this puzzle are:

* A new `--environment` (or `-e`) flag. This can also be set with the `HUGO_ENVIRONMENT` OS environment variable. The value for `environment` defaults to `production` when running `hugo` and `development` when running `hugo server`. You can set it to any value you want (e.g. `hugo server -e "Sensible Environment"`), but as it is used to load configuration from the file system, the letter case may be important. You can get this value in your templates with `{{ hugo.Environment }}`.
* A new `--configDir` flag (defaults to `config` below your project). This can also be set with `HUGO_CONFIGDIR` OS environment variable.

If the `configDir` exists, the configuration files will be read and merged on top of each other from left to right; the right-most value will win on duplicates.

Given the example tree below:

If `environment` is `production`, the left-most `config.toml` would be the one directly below the project (this can now be omitted if you want), and then `_default/config.toml` and finally `production/config.toml`. And since these will be merged, you can just provide the environment specific configuration setting in you production config, e.g. `enableGitInfo = true`. The order within the directories will be lexical (`config.toml` and then `params.toml`).

```bash
config
├── _default
│   ├── config.toml
│   ├── languages.toml
│   ├── menus
│   │   ├── menus.en.toml
│   │   └── menus.zh.toml
│   └── params.toml
├── development
│   └── params.toml
└── production
    ├── config.toml
    └── params.toml
```

Some configuration maps support the language code in the filename (e.g. `menus.en.toml`): `menus` (`menu` also works) and `params`.

Also note that the only folders with "a meaning" in the above listing is the top level directories below `config`. The `menus` sub folder is just added for better organization.

We use `TOML` in the example above, but Hugo also supports `JSON` and `YAML` as configuration formats. These can be mixed.

Fixes #5422

1 files changed, 2 insertions, 2 deletions

diff --git a/magefile.go b/magefile.go
index cd873dd77..19485b2be 100644
--- a/magefile.go
+++ b/magefile.go
@@ -21,10 +21,10 @@ import (
 
 const (
 	packageName  = "github.com/gohugoio/hugo"
-	noGitLdflags = "-X $PACKAGE/common/hugo.BuildDate=$BUILD_DATE"
+	noGitLdflags = "-X $PACKAGE/common/hugo.buildDate=$BUILD_DATE"
 )
 
-var ldflags = "-X $PACKAGE/common/hugo.CommitHash=$COMMIT_HASH -X $PACKAGE/common/hugo.BuildDate=$BUILD_DATE"
+var ldflags = "-X $PACKAGE/common/hugo.commitHash=$COMMIT_HASH -X $PACKAGE/common/hugo.buildDate=$BUILD_DATE"
 
 // allow user to override go executable by running as GOEXE=xxx make ... on unix-like systems
 var goexe = "go"