Exposing re2::FilteredRE2

[uhop]

Apparently re2::FilteredRE2 (see filtered_re2.h and filtered_re2.cc greatly improves the performance on large strings when multiple regular expressions should be matched. But it requires providing a 3rd-party case-insensitive sub-string search engine.

It would be nice to expose this functionality in a way compatible with RE2 and RE2.Set.