Add holes support to L.Polygon.setLatLngs(). Fixes #1518

I've added support of holes to `L.Polygon.setLalLngs()`.

But there are several more problems with holes in polygons:
  * no public method to get holes (`getLatLngs()` returns only external ring)
  * no functions to modify holes similar to `addLatLng()`, `spliceLatLngs()`, etc.
  * no information about restrictions on holes: should they be strictly inside external ring, can they intersect, include each other, etc.