Create a markdown-it-py-template repository

[choldgraf]

We currently have a markdown-it plugin template to help others build their own markdown-it plugins. We've also got the mdit-py-plugins repository, which has a collection of Python plugins for the markdown-it-py package.

It might also be useful to have a template repository similarly for markdown-it-py in addition to these plugins repositories. This could make it easier for others (or ourselves) to quickly generate new markdown-it-py plugins.