DOC: embed  description of bibmatching into the __doc__ of the module
NF:  :groupby: for sphinx ext directives

All bib directives now should accept :groupby: option to group present entries. e.g.

    .. bibmatching:: example.bib
        :style: demo
        :filter:
           author: \bTheGod\b
           howpublished: \b(Talk|Poster)\b
        :groupby: entry_type, subtype, year
        :sort:

Should group all the entries first by entry_type (e.g. articles vs inbook),
subtype (custom field I introduced to categorize e.g. between talks and posters
in 'misc'), and year (here needs more work to provide reverse sorting, but
works otherwise)