[Feature request] Move plugins in-tree

[za3k]

I've been using python-markdown2, and they have an idea of "extras" which are included as flags. Basically the same thing as plugins. It works way better than in Turndown.

I think the reason is that, much like other systems like Linux, they keep development in-tree. Bugs like (#416 (comment)) could not happen with in-tree plugin development.

What do you think about importing and maintaining a copy the authoritative copy of plugins in-tree?

I realize as I ask this that, given the number of open PRs and issues, this might not be a positive move just right now, but maybe this request will make sense down the line when Turndown is a little more caught up on maintenance.