Produce protocol-relative Markdown images

:fork_and_knife: 

This is a stab at fixing #528. We're already using a custom Redcarpet renderer; this extends it to implement a Supermarket-specific image tag callback ([docs](https://github.com/vmg/redcarpet/#span-level-calls)). Unfortunately, as is noted in vmg/redcarpet#51, it's not possible to simply create the protocol-relative URL and call `super`.

With this approach I believe Supermarket will not display images from servers that cannot serve HTTPS traffic.