Handler is a GraphicsMagick imageserver.Handler implementation.

It processes the Image with the GraphicsMagick command line (mogrify command).

All params are extracted from the "graphicsmagick" node param and are optionals.

Params (see GraphicsMagick documentation for more information about arguments):

- width / height: sizes for "-resize" argument (both optionals)
- fill: "^" for "-resize" argument
- ignore_ratio: "!" for "-resize" argument
- only_shrink_larger: ">" for "-resize" argument
- only_enlarge_smaller: "<" for "-resize" argument
- background: color for "-background" argument, 3/4/6/8 lower case hexadecimal characters
- extent: "-extent" param, uses width/height params and add "-gravity center" argument
- format: "-format" param
- quality: "-quality" param