Grow input buffer on multi-line messages

Suggested by a user in some channel: grow the input buffer when typing/pasting multi-line/long messages.

I guess we could do that up to a certain limit, (3-5 lines?).

I don't especially want to disrupt the flow of instant messaging chatrooms (where messages are usually small enough and things can go quite quickly), but I agree it can be handy when pasting code or longer messages.