Explicity reexport MarkdownIt

[paw-lu]

Explicitly export MarkdownIt.

Adds MarkdownIt to __all__.

This:

1. Allows markdown_it to pass strict mypy type checking, specifically --no-implicit-reexport
2. Removes the need for ignoring F401, as flake8 detects the use in __all__

[welcome]

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[paw-lu]

I actually see this was attempted in #120—though it was a more drastic proposal. Feel free to close if this is just adding noise.

[chrisjsewell]

Now merged #120, but thanks for the PR!