Use built-in MDEx plugin handling

[halostatue]

When leandrocp/mdex#288 is released, most of #169's special handling of :plugins can be reverted as there is native support for :plugins in MDEx.new/1.

I've got a watch on when the release is made and will provide a PR resolving this at that time.

[halostatue]

Gah. Forgot to link #172 to this to auto-close it.