Split irc messages into chunks

From Zephyr:
   barnowl / irc-cutoff / kchen  2013-05-09 16:38  (Everyone Left Wheel Thru)
       This might be an IRC limitation.  RFC 1459 says:

       "IRC messages are always lines of characters terminated with a CR-LF
       (Carriage Return - Line Feed) pair, and these messages shall not
       exceed 512 characters in length, counting all characters including
       the trailing CR-LF. Thus, there are 510 characters maximum allowed
       for the command and its parameters.  There is no provision for
       continuation message lines.  See section 7 for more details about
       current implementations."

       (I didn't read the rest of the RFC to know how many other bytes are
       involved in sending a message.)

I chose 450 to be the default cut-off semi-arbitrarily; freenode seems
to allow up to about 463 characters (to channel #agda, with username
jgross).