Add support for regex search

[dkharms]

We plan to add support for searching using regular expressions on indexable fields of type keyword.

A few words about the implementation:

• We decided that the filter name will be re;
• The regular expression engine will be the regexp package from the Golang standard library. It offers rich functionality while being safe from ReDoS, since this engine does not support backreferences.

Example usage of the new filter for finding Russian phone numbers:

phone:re("(+7|8)\s?\(?\d{3}\)?\s?\d{3}-?\d{2}-?\d{2}")