Add a markdown output mode

[dbarnett]

vimdoc should be able to generate markdown output as an alternative to vim helpfile syntax. We'll have to figure out how relative links can work first.

Note: These wouldn't be intended to be checked into the plugin repository like the generated vim helpfiles usually are. It could be used to dump browseable online help, though.

[dbarnett]

@wsdjeg was interested in this for SpaceVim.

I expect it would be straightforward to implement, with one gotcha that there's no markdown spec. I'd be interested in supporting both GFM and https://github.com/hoedown/hoedown, ideally by just using a subset of markdown syntax supported by both rather than implementing different output modes.