Support facet.matches to cull facets returned with a regex

I recently ran into a case where I really wanted to only return the next level of a hierarchical facet, and while I was able to do that with a coordinated set of dynamic fields, it occurred to me that this would have been much much easier if I could have simply used PathHierarchyTokenizer and written

&facet.matches="/my/current/prefix/[^/]+$"

thereby limiting the returned facets to the next level down and not return the additional N levels I didn't (yet) want to display (numbering in the thousands near the top of the tree). I suspect there are other good use cases, and the patch seemed relatively tractable.