Added options to bib-due script to pass input file (-i filename.bib), to
display simple help (-h), to specify which field to filter by (-l
fieldname), to specify which values to retain (-r
'values|in|regex|groups'), until which date to show (-u 2020-05-13).
Parsing remains brittle, which makes -l and -r not very useful as of
now; the parser would just get confused with different fields being
present.
Updated script is a little more resilient, and can be used to filter by
and catch various fields, controlled with a static variable at the top
of the file for now. Still brittle, and breaks easily on bibtex file
changes.
