message handling: improve IRC format handling for received messages

Rather than stripping colors all around and keeping other format codes,
we only strip initial and final formatting before parsing the message.

We store the original, unstripped message in #logmessage() and a
fully stripped copy of the message in #plainmessage()

This means that most plugins will now have full formatting of arguments
preserved, while stupid IRC usage of formatting whole lines will not
interfere with bot usage. Plugins that need a fully stripped version of
the message can still access it.