Python code block gist format

[nocibambi]

When parsing a python code block, it generates a gist with .python format. However, Medium does not recognize this, so currently, I need to rewrite the format to .py by hand to make syntax highlighting to work.

[switowski]

I just bumped into the same problem with .python and .shell extensions. Any chance the correct extensions could be automatically detected, so it would save some manual renaming?

[FrancescoSaverioZuppichini]

PR for .python fix #30