Option for coffeescript output, switch deprecated grunt.util._ to lodash

Coffeescript output is false by default, can be enabled with `options.coffee`. Uses `js2coffee` to convert the built javascript string to coffeescript before saving the file to disk.

Also, as per [Grunt's docs](http://gruntjs.com/api/grunt.util#grunt.util._-deprecated), using `grunt.util._` has been deprecated in favour of installing lodash or underscore through npm and saving it in `package.json`.