extend get_tags to offer simplified dict as return

[bensteUEM]

currently get_tags returns a complete list of dicts. In order to increase complexity in https://github.com/bensteUEM/SongBeamerQS/issues/18 a simplified return dict could be helpful

rtype='original' -> passthrough rtype='id_dict' -> {'9':"tag_name", ... } rtype='name_dict' -> {"tag_name":'9', ... }