✨ Allow to be run as a module

[alexandrehassan]

The changes allow advent-of-ocr to be called as a module. To achieve this the code has been moved out of __init__.py. This also adds a cli for the the tool that can be called after it has been installed.

>  python -m advent_of_code_ocr -h
usage: __main__.py [-h] [-f FILL_PIXEL] [-e EMPTY_PIXEL] [input_text]

positional arguments:
  input_text            Input text

options:
  -h, --help            show this help message and exit
  -f FILL_PIXEL, --fill-pixel FILL_PIXEL
                        Fill pixel character
  -e EMPTY_PIXEL, --empty-pixel EMPTY_PIXEL
                        Empty pixel character

Also included test to make sure argument parsing works as intended.

Additional cli tests:

echo -e '.##..###...##.\n#..#.#..#.#..#\n#..#.###..#...\n####.#..#.#...\n#..#.#..#.#..#\n#..#.###...##.' | python -m advent_of_code_ocr
> ABC

cat test.txt | python -m advent_of_code_ocr -f b -e a
> ABC

Closes #36