WIP: 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 such as the bounding box of the detected text.

TODO:
* Add documentation to match_text
* Add documentation to TextMatchResults
* Add test for no match
Print hOCR XML if parsing fails

For easier debugging