exports.py: write warnings to stderr, not stdout (which might be muted)
Merge pull request #20 from mk-pmb/warn_on_stderr

exports.py: write warnings to stderr, not stdout (which might be muted)
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