Add new "allowOverflow" option

Add new "allowOverflow" option that respects the minimum font pixel size, but won't call the fail callback if the text doesn't fit in the specified container. Basically the text will stay at the minimum size, but will overflow vertically. It's up to the user to set appropriate styles on the HTML elements to hide the overflow, if desired.

This may or may not also solve Issue #23: 
https://github.com/jquery-textfill/jquery-textfill/issues/23