Superscript/Subscript Implementation in Markdown (BB-8686)

I'd like to propose the addition of superscripts and subscripts to the flavour of markdown the Bitbucket uses. Since Bitbucket uses python-markdown, there's a plugin to enable superscripts with the markdown syntax of ^{<superscript>}.

I'd suggest implementing subscripts as ^^<subscript>^. Subscripts and superscripts are supported with Creole as the markup for wiki pages, but that support isn't there for markdown. It would really aid with writing technical wikis with footnotes.