[checkoutlinesufo] consider incoming UFO formatVersion

[frankrolf]

As indicated in my UFO3 issue here, checkoutlinesufo follows somewhat of a hybrid approach:
It writes UFO3-style layers into UFO2 files.
If I remember correctly, the UFO version number is manually changed back to 2 after writing the UFO3 layer with defcon (see here: https://github.com/adobe-type-tools/afdko/blob/develop/afdko/Tools/SharedData/FDKScripts/CheckOutlinesUFO.py#L143-L153)

checkoutlinesufo fails when run on a real UFO3 file:

Saving font...
Traceback (most recent call last):
  File "/usr/local/bin/checkoutlinesufo", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python2.7/site-packages/afdko/Tools/SharedData/FDKScripts/CheckOutlinesUFO.py", line 1146, in main
    run(sys.argv[1:])
  File "/usr/local/lib/python2.7/site-packages/afdko/Tools/SharedData/FDKScripts/CheckOutlinesUFO.py", line 1135, in run
    font_file.save()
  File "/usr/local/lib/python2.7/site-packages/afdko/Tools/SharedData/FDKScripts/CheckOutlinesUFO.py", line 139, in save
    writer = UFOWriter(self.defcon_font.path, formatVersion=2)
  File "/usr/local/lib/python2.7/site-packages/ufoLib/__init__.py", line 581, in __init__
    raise UFOLibError("The UFO located at this path is a higher version (%d) than the version (%d) that is trying to be written. This is not supported." % (previousFormatVersion, formatVersion))
ufoLib.UFOLibError: The UFO located at this path is a higher version (3) than the version (2) that is trying to be written. This is not supported.

It would be great if checkoutlinesufo could recognize the incoming file format, and adjust its behavior accordingly.