Generalises the two bits of the program that will be required either
way (extension parsing and syntax loading).
A dependency on Comrak is introduced as I think GitHub-flavoured
Markdown (with all its fancy extensions) is desirable!
The first step with this tool will be to use it as a source-filter for
cgit. The second step is to use it as the Markdown renderer by
depending on one of the Markdown libraries, with integration for
rendering code snippets directly.