OptiFine support

While jarmods still are quite a while away, OptiFine provides itself as a tweaker. We've had documentation on the wiki about how to use this for a while, but it's still not easy, and many people have requested OptiFine support.

Because of a lack of a remote version endpoint the user will have to download OptiFine manually and then select it from the version selection dialog. This should still be way easier than following the wiki article.