Change markdown to markdown_github for pandoc invocation

[TeMPOraL]

The current way quickdocs renders README creates ugly-looking text blobs in case of some projects hosted on Github. Compare, for example, closer-mop README on Github and its rendering on QuickDocs.

Since most (if not all) Markdown readmes come from GitHub anyway, I think it would be good to default to markdown_github instead of markdown format, which will result in clean READMEs for most of the (Markdown-using) projects.

See also: pandoc docs.