Attribute overflowLength to inhibit Emoji processing

It might happen, if the text to process is very large, that putting all those spans for the emojis will hang
the user interface. This new attribute allows you to define a limit in characters that the text must have in
order to be allowed to process emojis.

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Sure, that's even better. I'll make another patch and void this one.