Ability to set basePath

Allows theme-specific basePath settings, so that all themes do not have to reside in the same folder.

Example config:
    '^foo' => array(
        'name' => 'theme-name',
        'basePath' => 'vendor.foo',
    ),
    '^bar' => array(
        'name' => 'another-theme',
        'basePath' => 'vendor.bar',
    ),