Add ability to 'like' and 'unlike' themes and maintain list of favorite themes

I really wanted to ability maintain a small set of themes I like and randomize through them.  It could be as easy as removing those themes that I don't like, but then you'd might have to be careful when merging changes and collaborating, etc.

Instead, I thought just creating a simple shell function to make a symlink in another directory would be an easy way to keep track of 'liked' themes.  This way there isn't really any overhead because the themes aren't be copied, and removing favorites or 'unliking' themes is as easy as remove the symlink.

Goes along with Issue #397