Support for ETC_RGB4? #16

Traceback (most recent call last):
 File "/usr/local/bin/unityextract", line 146, in <module>
 main()
 File "/usr/local/bin/unityextract", line 143, in main
 exit(app.run())
 File "/usr/local/bin/unityextract", line 56, in run
 self.handle_asset(asset)
 File "/usr/local/bin/unityextract", line 127, in handle_asset
 image = d.image
 File "/usr/local/lib/python3.5/site-packages/unitypack/engine/texture.py", line 151, in image
 raise NotImplementedError("Unimplemented format %r" % (self.format))
NotImplementedError: Unimplemented format <TextureFormat.ETC_RGB4: 34>