Support existing `current_` methods and extras

setup auto discover for a valid current_ themeable instance. A bit of
magic to ease integration. Options are:
  - configure the current_* method to use
  - use the configured `themeable_class` to try
     `current_<themeable_class>` method
  - try to search params object for `params["#{themeable_class}_id"]`
  - fallback to default resolver [which looks in app/views]

Add an option to specify which order the resolver is used. Options are:
  - :prepend   - Use the theme_resolver before falling back to app/views
     This allows you to gradually migrate into theme_renderer (default)
  - :overwrite - Replace the current _view_path with our resolver.
  - :append    - Use theme_resolver as the fallback resolver. This
    allows you to perform a gradual move away from theme_renderer.