starbound.open_file: drop superfluous var, add file extension aliases

The shorter filename extensions aren't used by the game, but in my symlinks.
starbound.open_file: allow overriding file type guessing
Expose fext2fmt lookup table
Merge @mk-pmb's changes into latest master

I updated the changes to fit the rest of the code style in this project.

Conflicts:
	starbound/helpers.py