Support for right-aligned text

When text is aligned to the right, the popup shows to the left of the label and this is not necessarily the left of the actual text in the label.

I've added a new setting pushPopupToLabel which makes sure the popup location is left from the actual text.
This is implemented by placing the first character of the label in a span with class "KeyTips__startLabel" (can be modified using settings.startLabelTag and settings.startLabelClass) and then determine the popup location base on this class.