Add (optional) 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.

This improves compatibility with the *biblatex* package which uses entry types such as `mvbook`.