Add config dir support

[bep]

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).

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.

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