Adding .skim as a file format

Added the .jst.skim file format (https://github.com/jfirebaugh/skim) to the fileTypes array. 

_Skim is the Slim templating engine with embedded CoffeeScript. It compiles to JavaScript templates (.jst), which can then be served by Rails or any other Sprockets-based asset pipeline._
Merge pull request #32 from tommyp/patch-1

Adding .skim as a file format