Region: Add convenience methods

Sometimes it's convenient to think in terms of width and height, other
times in terms of extents.  The addition of `right`, `bottom` and
`from_extents` to `Region` allow either model to be used as convenient.

These and the `_bounding_box` helper function will be useful for more
advanced uses of OCR which requires manipulating regions.
OCR: Add stbt.match_text()

`match_text` is intended to be used like `detect_match`, etc, but for
text.  You search for a specific piece of text and it returns extra
information including the bounding box of the detected text.