Tests for prefer_specials

Will ensure prefer_specials is being obeyed correctly.
Support for prefer_specials in SeriesParser

If prefer_specials is True, an entry matching both a normal id type
and special will be flagged as a special. If false, entry will be
flagged as the normal id type.

E.g.: title: the show s03e04 special
prefer_specials = True -> series_id_type: special
prefer_specials = False -> series_id_type: ep
Add prefer_series support to series

Series now accepts an option 'prefer_specials'. If true, entries in
that series which parse as both a normal id type (ep, etc) and a
special will be flagged as specials. If false, they will be flagged as
the normal id type.
prefer_specials support for configure_series

Also allows plugin to accept list of special_ids