Add -o option to limit to certain Ruby versions.

I added this to allow me to easily run tests for my projects against a couple of major ruby versions (1.8, 1.9) without having to remove a bunch of other rubies (ree, 1.8.6, etc.).

Depends on the whatis plugin to simplify version specification. Let me know if that's a hurdle for inclusion; if so, I can make it optional.