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

hugolib/paths/themes.go

@@ -20,7 +20,6 @@ import (
 	"github.com/gohugoio/hugo/config"
 	"github.com/spf13/afero"
 	"github.com/spf13/cast"
-	"github.com/spf13/viper"
 )
 
 type ThemeConfig struct {
@@ -73,18 +72,11 @@ func (c *themesCollector) add(name, configFilename string) (ThemeConfig, error)
 	var tc ThemeConfig
 
 	if configFilename != "" {
-		v := viper.New()
-		v.SetFs(c.fs)
-		v.AutomaticEnv()
-		v.SetEnvPrefix("hugo")
-		v.SetConfigFile(configFilename)
-
-		err := v.ReadInConfig()
+		var err error
+		cfg, err = config.FromFile(c.fs, configFilename)
 		if err != nil {
-			return tc, err
+			return tc, nil
 		}
-		cfg = v
-
 	}
 
 	tc = ThemeConfig{Name: name, ConfigFilename: configFilename, Cfg: cfg}