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>
Should we parameterize `index` and `length` for `EmojiconHandler.addEmojis`? Let's say `EmojiconHandler#addEmojis(context, text, emojiSize, index, length)`. Then we can allow the text to be processed from `index` until `index+length` instead of ignoring the whole text.