Add support for linking to GitHub comments

[sloria]

[webknjaz]

Would the syntax be something like

:issue:`123#issuecomment-12345`

?

Or does it need to be a new RST role?

[webknjaz]

FWIW here's what I do in my projects typically —https://github.com/aio-libs/aiohttp/blob/a379e63/docs/conf.py#L158-L164 (pending replacement with this extension). I have a generic :gh: role as a stop gap for anything not covered by the specific ones like :pr: or :issue:.

[sloria]

I like the proposed syntax since it would support any hash fragment within an issue or PR 👍