Add contains_regex(regex_pattern) matcher

[davified]

Heya @mwilliamson , thanks again for the great work on this library.

This PR adds a new matcher contains_regex(...), as requested in #16. For example:

assert_that("2020-01-01", contains_regex(r'[\d]{4}-[\d]{2}-[\d]{2}'))

I've added unit tests for the new method, and all the unit tests are passing

Keen to hear your thoughts/feedback. Cheers :-)