Unify code to correct file names in coverage output

[kategengler]

From #58

There is a place that changes the displayed filenames in the instrumenter: https://github.com/kategengler/ember-cli-code-coverage/blob/master/lib/coverage-instrumenter.js#L27

Is it possible to do this massaging there?

Another potentially relevant spot: https://github.com/kategengler/ember-cli-code-coverage/blob/master/index.js#L129