Add priority sorting

[liabru]

In complex cases it can be required to apply matchHeight to elements in a certain order e.g. #104 to get the right result.

It seems that selecting in a certain order won't help as selections are always DOM ordered. So you must make multiple selections, but this can still be tricky to guarantee the correct order.

A possible solution is to add a priority value to groups / elements and then sort $.fn.matchHeight._groups and the elements before applying.

Another solution is to apply matchHeights iteratively until no more elements change height (i.e. global solution found), but this might be slow or unstable.