Added onTruncate init parameter / success function

onTruncate can be passed at initialization time. It is a function that will
fire if trunk8 *does* actually truncate the text. If trunk8 *does not*
truncate the text, the passed function will not fire.

This is useful if the front-end needs to be modified / decorated in some
custom manner when trunk8 successfully truncates text but left alone
when there is no truncation.