Added support for plain text and Markdown files.
Moved `*_test.py` handling next to `*.py` files.
Colorize status of each test (green/red).

[PASSED] shows up as green while [FAILED] shows up as red assuming the output is
a tty; if not (e.g., output redirected to a file or pipe), it will be displayed
without any colors.
Fix test which should be using MIT license.