Add 'join' option to concatenate scripts prior to compilation

This change allows for the concatenation of CoffeeScript files prior to compilation (in a fashion similar to the CoffeeScript compiler). This is achieved by setting the newly introduced 'join' option to true. 

This is in advance of adding sourceMap support. When concatenating multiple CoffeeScript files into a single JavaScript file using this plugin, sourceMaps will be irrelevant for files concatenated after compilation. The 'join' option paves the way for generation of a relevant sourceMap.