GraphicsMagick support?

[ScenK]

As gm support not only GraphicsMagick but also ImageMagick for its base module, it that possible to make this module support GraphicsMagick? That will be a great help!

[twolfson]

Yea, we are open to supporting GraphicsMagick. In fact, #33 was working on this but it turned out to be the wrong solution to a different problem. The PR is rather incomplete but it's a good starting point for parameters and edge cases to handle