Improve Python format

[rossant]

• Renamed _cli() to convert_files()
• Closes Improve Python format #28.
• Closes Bug in Python format with docstrings #31.
• Closes Commented Python code should not be interpreted as Markdown cell #41.

[coveralls]

Coverage remained the same at 90.34% when pulling 466c344 on update-python-format into b5849b6 on master.

[coveralls]

Coverage increased (+0.14%) to 90.49% when pulling cb275c3 on update-python-format into b5849b6 on master.

[coveralls]

Coverage increased (+0.21%) to 90.55% when pulling b7b714b on update-python-format into b5849b6 on master.

[coveralls]

Coverage increased (+0.3%) to 90.64% when pulling 902002c on update-python-format into b5849b6 on master.