Should support parts parsing for plenum protocols

We should support the same functionality as the one that exists for committee meeting protocols ([see source here](https://github.com/hasadna/knesset-data-python/blob/649e109002a2ef4e6dad484dbaaa3454e673abbe/knesset_data/protocols/committee.py#L65)).

This will be used by the knesset-data-pipelines project for parsing plenum meeting protocols ([see issue here](https://github.com/hasadna/knesset-data-pipelines/issues/68)).

Code can and should be based on existing parts parsing from the knesset-data-django project ([see source here](https://github.com/hasadna/knesset-data-django/blob/master/knesset_data_django/committees/plenum_protocol_parts.py))