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

common/maps/maps_test.go

@@ -16,6 +16,8 @@ package maps
 import (
 	"reflect"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestToLower(t *testing.T) {
@@ -70,3 +72,52 @@ func TestToLower(t *testing.T) {
 		}
 	}
 }
+
+func TestRenameKeys(t *testing.T) {
+	assert := require.New(t)
+
+	m := map[string]interface{}{
+		"a":    32,
+		"ren1": "m1",
+		"ren2": "m1_2",
+		"sub": map[string]interface{}{
+			"subsub": map[string]interface{}{
+				"REN1": "m2",
+				"ren2": "m2_2",
+			},
+		},
+		"no": map[string]interface{}{
+			"ren1": "m2",
+			"ren2": "m2_2",
+		},
+	}
+
+	expected := map[string]interface{}{
+		"a":    32,
+		"new1": "m1",
+		"new2": "m1_2",
+		"sub": map[string]interface{}{
+			"subsub": map[string]interface{}{
+				"new1": "m2",
+				"ren2": "m2_2",
+			},
+		},
+		"no": map[string]interface{}{
+			"ren1": "m2",
+			"ren2": "m2_2",
+		},
+	}
+
+	renamer, err := NewKeyRenamer(
+		"{ren1,sub/*/ren1}", "new1",
+		"{Ren2,sub/ren2}", "new2",
+	)
+	assert.NoError(err)
+
+	renamer.Rename(m)
+
+	if !reflect.DeepEqual(expected, m) {
+		t.Errorf("Expected\n%#v, got\n%#v\n", expected, m)
+	}
+
+}