Feature: fail_with matcher

It would be useful to have a `fail_with` matcher similar to [RSpecs `fail_with`](http://www.rubydoc.info/gems/rspec-expectations/RSpec/Expectations.fail_with).

Sometimes we have to write some more complex tests which expect certain underlying behaviour from the tested code which is hard to cover with the existing expectations. On some occasions a custom assertions helps but on others such an assertion would be overkill.

Having a simple option to fail a test in an expected manner without raising an arbitrary error would increase test readability a great deal.