office: create policy for what to do about unknown members

previously, encountering an unknown member meant that any parser of
this type would abort.

now, the user can set parser.unknown_member_policy to either 'omit' or
'keep' if they don't want the current action of 'abort'

note that this causes pylint to complain about branching depth for
remove_all() because of the nuanced error-handling.  I've disabled
this check.