Added option to truncate text

New per-field option to truncate text if it's too long instead of replacing it
with it's old value. This especially makes a difference for long, pasted text
fragments.

This patch was created by Martijn Geerts (De-Fecto) from the Processwire forum:
http://processwire.com/talk/topic/2343-char-counter-for-texttextarea-fields/?p=47468