Add "Global Blacklist" to settings form

I've added a simple Blacklist of terms to the settings form. These terms are merged with the $blacklist array specified (if there is one). This seems like a good way to get rid of keywords that keep coming up that I don't ever want (like "nbsp" and "quot" and "amp").

Probably not the most efficient piece of code ever written. If you wanted to Blacklist a lot of words, there's probably a better way.