add gtk backend

I set up a GTK backend for Julia, since I think it's dependencies are simpler (no wrapper, no internal API). These files add support for it.

To allow the user to pick their backend, I added the following options:
Winston.ini added `[default] output_surface = "tk" or "gtk"`
or override with
`ENV["WINSTON_OUTPUT"] = "tk" or "gtk"`
I'm not sold on the names, so feel free to change them when you merge