Perform template field substitution on albums

- adds another traversal through all plugins' template_fields for each
  'evaluate_template' call.
- requires the following idiom (or equivalent):

    @Plugin.template_field(field')
    def _tmpl_field(album):
        """Return stuff.
        """
        if isinstance(album, Album):
            return stuff
Merge pull request #274 from pedros/album_template_fields

Perform template field substitution on albums