Allowing use of GraphicsMagick as conversion tool

[wcgallego]

Hey! Thanks for putting this together - caught your Velocity 2016 talk and was psyched to implement at my shop.

We're using GraphicsMagick over here so I added an option to use such. Not hung up on the implementation, just 'worked'.

I also pulled out the conversion to png and base64 for the input image from the while loop as it was unnecessary to do it on each iteration, shaving off a few ms.