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:
* [x] Convert tests that say `stbt.detect_match().next()` to use `stbt.match()` directly
* [x] Write test for passing a frame to `stbt.match()`
* [x] Write test for `stbt.match(region=...)`
* [X] Squash
* [x] Improve commit messages
* [x] Document `region` parameter
* [x] Add `match` to `stbt.__all__` after merging #218.