Allow setting system content on reset

[gparrello]

Adding a new argument to reset_chat_history, allowing to pass content to it. Content still defaults to the ASSISTANT_PROMPT environment variable when not set.

[n3d1117]

Interesting approach, thanks @gparrello! I like the idea of combining it with the reset command.

The docs say:

gpt-3.5-turbo-0301 does not always pay strong attention to system messages. Future models will be trained to pay stronger attention to system messages.

So I guess this could become even more relevant with gpt-4 model! I'll merge this soon