Ocr 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:

- [x] Document `match_text`
- [x] Document `TextMatchResult`
- [x] `match_text`: Test failure to match case
- [x] Install `python-lxml` dependency (Fedora & Debian packages)
- ~~Address segmentation (?) / thresholding (?) issue with real-world UIs~~