API: Add stbt.match

Adds new API `stbt.match` which was previously conspicuous in it's absence.  This, in combination with the ability to treat `MatchResult` as a boolean makes it much easier to compose matching operations in user code.  e.g.:

    assert stbt.match('template.png') or stbt.match('template2.png')

TODO:
* [ ] Tests
* [ ] Squash
* [ ] Improve commit messages