Moves most of the logic previously in cli.py to convert.py, for usage by
other submodules. Adds Container.to_json and Container.to_csv. Makes
adjustments/fixes to other parts of the library, based on edge-cases
encountered (such as infinite recursion in anntations).
- Fixes `.extract_words`, which had been returning incorrect results when `horizontal_ltr = False`
- Fixes `.resize_object`, which had been failing in various permutations
- Brings utils.py test coverage to 100%