Add option to pass a custom mustache template to json2css

A custom template file can be specified for json2css to use.

Because all of my sprites are in the same spritesheet, I would like to use [Stylus' @extend directive](http://learnboost.github.io/stylus/docs/extend.html) to have all sprites extend from a `$sprite` placeholder class rather than use a mix-in.

This also allows for greater customization of the variable names. I use dashes in my Stylus variable names; with the custom template I can keep the sprite variables consistent with my other variables.