Add `--output` option to `font-reduce.py`.

Add `--output` option to `font-reduce.py`.

This allows to to reduce system fonts, where you don't have write
permission on the source directory.  For example, I was trying to
reduce GoodCityModern:

  $ font-reduce.py /usr/share/fonts/freefonts/gocmase_.pfb
  Save Failed
  Traceback (most recent call last):
    …
    File "…/font-reduce.py", line 65, in convert
      o.generate(output_file, flags=('PfEd-comments',))
  EnvironmentError: Font generation failed

Which is a cryptic error message for a `permission denied` error.  The
newly supported working equivalent is:

  $ font-reduce.py -o GoodCityModern.woff /usr/share/fonts/freefonts/gocmase_.pfb