Hook handlers could provide Dredd with a list of supported features

[honzajavorek]

We may want to introduce new features to the dredd-hooks-template in the future. Hook handler implementations might get into situation when the new features are not available (missing implementation, programming language limitations, etc.), while Dredd requires them in its newer versions.

We could introduce a way Dredd could ask the hook handler about features it implements and hooks would reply with their specification. See @w-vi's comment: apiaryio/dredd-hooks-template#17 (comment) Older hook handler implementations wouldn't reply with correct message back and would be treated according to some default values.

I propose the features could be somehow connected with tests in the dredd-hooks-template, so if hook handler declares it supports certain feature, it essentially means it passes certain test in the Gherkin-based BDD test suite.