Added back the highlight filtering, and refactored it.

I do not know why this was removed. I use it all the time, so I decided to re-add it and refactor to only what's necessary.

I have added documentation that I have forgotten to write so it can be added to the wiki. I cannot use Textual without this as I rely on it with all the IRC channels I am in.

Hope this gets added back in,
Thanks for a great IRC Client.