Fix broken vimwiki links in pandoc

[ajkavanagh]

So to workaround a missing feature of Pandoc's markdown reader, I implemented a post converter that took text like [[link]] and converted it to a link.

However, the markdown reader converts [[actual-recognisable-link]] into '[[', Link(...), ']]' which breaks my converter.

What really needs to happen is that the Pandoc markdown reader grow the ability to handle a vimwiki link and thus I won't need my post document pandoc processor to convert the str to a link. It seems like the better solution.