[help][core] Multi-line help

```.help``` will now display multi-line docstrings as several messages, one for each line, instead of concatenating them into a single long one.

Those that consist of more than a certain number of lines (including the example for commands that have one) will be PRIVMSG'd to the asker instead of being posted into a channel. This number (3 by default) is configurable via the [help]/threshold config attribute.