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.