Add support for non-standard entry types

Add an additional optional ignore_nonstandard_types argument to
BibTexParser.__init__(), assign its value to a property of the same name
and use it to determine if records with non-standard entry types should
be skipped in _parse_record(). The default is to ignore non-standard
entry types.
Merge branch 'feature/nonstandard-entry-types'

Add (optional) support for non-standard entry types.