Add a "prevent these words from being sent" config

[iphydf]

I'd like a list of strings that must never be sent through toxic. If I try to send it, toxic should block the enter key's action (not send, not clear the message entry line either). This is to prevent leaking certain well-known information, e.g. hunter2. It should be a simple substring match, so btwhunter2 should also be blocked. If we want to be fancy, we can do regex, but then we need to start depending on a regex library, so I'd start with just a substring match.