Update the plugin to support emitSuccess for when a linting succeeds.

I have also added my name as a contributor, updated the README to reflect said changes and added a unit test for emitSuccess.

Use Case:
if a user needs to trigger other grunt functions based on a successful lint. - This is especially handy when a a rather large set of SCSS files are being linted.

Other thoughts:
This is a good companion to emitError