Support conversation controls / oob commands

Add some form of support for conversation controls or out of band commands to modify a conversation. Some option to provide a different answer, re-run the prompt, etc.

Feature ideas:
- [ ] Update (force update if bot wasn't alive)
- [ ] Redo (redo last reply via edit or intelligent history chaining)
- [ ] Step mode (only let the bot run when you ask it to update, good for multiuser)
- [ ] System prompt (add a system prompt instead of user)

This is a bit more complicated, as either we'll have to support message editing or more intelligent history parsing. If we do that we could support continuations though maybe

It is possible these could be implemented as slash commands or right click commands depending on how those get implemented.

Maybe we could do something that involves tagging the bot again
```
@bot prompt
response 1
reply A
response 2
@bot !redo
new response 2
reply B
[bot unresponsive]
@bot !update
new response 3
```