Allow to specify the maximum length of a line with the new split lines feature

This adds a new setting to irssi called "split_max_length" as it existed on splitlines.pl.

It allows the user to define the max length of the messages before they are split.
If it is set to 0 (the default) it will use the default split lines algorithm to find the max_length.

This is an addition to Pull Request #29.
This needs to be added to irc_server_split_action() also... or maybe it's better if this is checked inside split_line() instead? split_line() is called by both split_message() and irc_server_split_action().