Add custom highlight words

This PR allows users to specify any number of words which they should be highlighted on. Here's a typical usage:

![selection_012](https://cloud.githubusercontent.com/assets/426222/5488098/9faa7c5e-86b9-11e4-814a-fbbc947f3176.png)

And then it behaves just like a regular nick mention:

![selection_013](https://cloud.githubusercontent.com/assets/426222/5488100/a147227e-86b9-11e4-9d84-949db4dc5b5f.png)

The only question I have here is what to do about the translations? I added the string to en-gb, but none of the others.