Fix the rewriters.

[MattiasBuelens]

9fb0b0e disabled the rewriters for now, but these should be fixed and re-enabled eventually.

Ideally, the original message is preserved and the rewriters are re-evaluated on all messages when changed. If that's not possible or too expensive, restoring the old behaviour (using the currently active rewriters on new messages only) will do. Either way, more research is needed in order to figure out where and how this should be hooked into Twitch's code.