Add --force-filetype option to force a filetype for an instance file

[mschoettle]

Add --force-filetype option to force an instance file to be parsed with the requested file format parser.

Fixes #341

[sirosen]

Hi, thanks so much for putting together this changeset!

There are some very minor tweaks I'd like us to make before merging, but I think this is good. And I agree that --force-filetype has a nice symmetry with --default-filetype, so that's a nice name for this option.

[sirosen]

I think it might make more sense to use force_filetype as an alternative to calling path_to_type().
e.g.,

if force_filetype is None:
    filetype = path_to_type(path, default_filetype=default_filetype)
else:
    filetype = force_filetype

I'm pretty sure it doesn't matter in practice -- outside of unit tests, the missing support block is hard to reach -- but IMO it would read cleaner because we take care of the assignment before we start making any control-flow decisions based on the value.

[mschoettle]

Good point! This actually caught a potential issue when force_filetype is not in self._by_tag.

[sirosen]

Thanks again for going after this!