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 to have all sprites extend from a $sprite placeholder class rather than use a mixin.

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.