Enable optional tap through

This is useful if you have a cell with a TTTAttributedLabel covering the entire contentView and a small link somewhere in the text. You want to be able to tap the link, so userInteractionEnabled must be true for the entire label, but you also want to be able to tap outside of the link and select the cell and perhaps trigger a segue.

Setting allowsTapThrough to true checks if the user tapped a link, and if not it ignores the touch and lets the next view handle it.