Alias support 

Adds the support as requested in #31.

A quick example; copy the `test.js` file to `test.example` then run the following;

`sloc spec/testfiles/test.example -a example=js`

I noticed there was no tests for `cli.coffee` so I didn't add any.

Any questions or problems let me know and I'll sort them out :+1: - cool project BTW.