Make configFile optional

[geddski]

When using Grunt it's common to put all project config into the single Gruntfile. My gruntacular grunt plugin makes this especially easy, even allowing devs to override settings in their testacular.conf.js. It would be nice if the configFile was optional, for those projects wanting to put all their settings in the Gruntfile. Currently you can make an empty testacular.conf.js and that works but isn't ideal.

[geddski]

rock on! Thanks @vojtajina