Option to ignore WP Core

Scratching my own itch here (issue #24) and added an option to ignore WP core as part of the Yeoman setup. This is my first time playing with Yeoman generators, so I'm not 100% sure I've done things the 'right' way, I just had a look at what was already there, added to it, modified it, and tested it. Hope I've done it somewhat right, though :)