Add support for WordPress languages

Hi,
I want to add support for WordPress languages. I want to be able download languages in my projects. Example:
```
{
  "repositories": [
    {
      "type": "package",
      "package": {
        "name": "wordpress",
        "type": "wordpress-core",
        "version": "3.8",
        "dist": {
          "type": "zip",
          "url": "https://github.com/WordPress/WordPress/archive/3.8.zip"
        }
      }
    },
    {
      "type": "package",
      "package": {
        "name": "pl_PL",
        "type": "wordpress-language",
        "version": "3.7.1",
        "dist": {
          "type": "zip",
          "url": "https://github.com/kierzniak/wordpress-languages/archive/pl_PL.zip"
        }
      }
    },
    {
      "type":"composer",
      "url":"http://wpackagist.org"
    },
  ],
  "require": {
    "php": ">=5.3.0",
    "johnpbloch/wordpress-core-installer": "~0.2",
    "composer/installers": "~1.0",
    "wordpress": "3.8",
    "wpackagist/advanced-custom-fields": "*",
    "pl_PL": "3.7.1"
  },
  "extra": {
    "installer-paths": {
      "content/plugins/{$name}/": ["type:wordpress-plugin"],
      "content/languages/": ["type:wordpress-language"]
    }
  }
}
```