Implement the custom prefixes module

I've tested it and it seems to work fine. Though there are a number of issues that I'm not sure how to work out.
- txircd parses the modes a module implements before calling `module.load()`. Since I depend on what gets loaded in to be able to tell what modes I'm returning I'm not sure how to work around this.
- Because txircd does not refresh its modes when rehashing modules I can't think of a good way to work config changes into this module.