Feature Request: Timeout

[chunleng]

As title, sometimes ai.vim takes a really long time and returns error, sometimes it seems to break out of the loop and return a really long but repetitive result which I cannot use.

However, I might have already proceed on to other stuff and most likely do not need the result anymore, so I timeout with the user's preference might help a lot with the situation.

[aduros]

Sounds like a good idea, I would accept a PR for this if you want to take a crack at it 😄