Enable config for markdown extensions

[ZedThree]

Some projects specify markdown extensions with config, e.g. markdown.extensions.toc(anchorlink=True). This currently doesn't work, I'm not sure when it stopped -- could be a markdown version thing.

It should be pretty easy to extract the arguments to a dict to pass in via the extensions_config argument to markdown.Markdown

[d7919]

@ZedThree is this fixed now given v7 is out?