Add support to format "continuation lines"

[wasamasa]

Screenshot

An user posting more than one line in a row could be formatted in an alternative manner to indicate that they're possibly going for a lengthy monologue until they're interrupted by another user.

As for the implementation, this would require an extra format string. If it's nil, circe-format-say should be used, otherwise normal formatting rules would apply to it. Not sure how continuation lines could be detected...

[jorgenschaefer]

I like this.