Package path declaration

This may be a better approach to what I was trying to achieve in my WordPress package pull request.  Certain cases may be flexible and allow names/locations to be changed.  This patch in my case for WordPress allows a custom content directory to be declared, as well as allows the composer root to live outside of the wordpress directory.  

``` json
{
    "extra": {
        "package-paths": {
            "wordpress/wp-content/plugins/{$name}/": ["wordpress-plugin"],
            "wordpress/wp-content/themes/{$name}/": ["wordpress-theme"]
        }
    }
}
```

Can install all wordpress plugins and themes into the wordpress directory where the actual installation lives, opposed to maintaining an array of each of the package types in the install-paths array along side of the regular requirement declarations.

This time not limited to only wordpress.