Added ability to specify feature file and line numbers from command line

Hi,

I've made a small enhancement to RuntimeOptionsBuilder to allow the specification of which feature file (and optionally line numbers) to run. It seems that this functionality was present in the old addCliFilter(RuntimeOptions) method, but was lost when moving to the new method (I assume because it didn't fit with the style). I've tried to implement it in keeping with how you've done the tags. I've also updated the README to include a section on what can be specified from the command line.

Example usage:

    grails test-app :cucumber features/search.feature:10:20