Make bugz modify --fixed/--invalid configurable

[jbeich]

bugz modify --fixed and bugz modify --invalid hardcode STATUS and RESOLUTION. Some instances of Bugzilla may rename them to better match their workflow.

It'd be nice if pybugz.d/*.conf could specify alternative names similar to search_statuses e.g.,

[FreeBSD]
resolved_status = Closed
invalid_status = Rejected