Regex filter on terms_stats facet

Hi ES Team,

Anyone could tell me if you will accept and merge my pull request :

If would really like to have it in 0.20 for my customer without having to build
my own 0.20 version.

Could you tell me if I must modify something or if there's something bad in this
pull request?


BTW, if you merge it, don't forget the relative documentation pull request :

Thanks for letting me know.

David Pilato
Twitter : @dadoonet / @elasticsearchfr / @scrutmydocs