Add basic commands to reset the history

[PaulWoitaschek]

Currently, - if I understand correctly - the whole history of messages gets transmitted for the chat completions.
This is problematic because sometimes the context changes:
You might first ask the chat bot for a recipe on how to make a good paella. And then at a later point want to know about the pros and cons of an FSME vaccination.

It would be great if we had the option to reset the history. This could for instance be done with a command like:
!new

That would also have the additional benefit of lowering the costs as there are less tokens (unnecessarily) transmitted.

[cycneuramus]

For the OpenAI API, the transmitted history is actually limited to 5 messages (see the max_history param). Hard-coding the value like this is obviously not the best idea, but at least the API costs will be kept somewhat in check.

Fixing this, and making it possible to reset the history, is a good idea, but I am unfortunately pressed for time to keep up with this project ATM.