Support github shorthands

[gkatsev]

Currently, we have to copy and paste the url but sometimes you don't have the link handy and it's easier to be told the org/repo#issue shorthand so that we can do something like:

github: w3c/webvtt#460

instead of doing the full

github: https://github.com/w3c/webvtt/pull/460

Ties into #45 as well, with automatic topic.

[dbaron]

Seems like a good idea for all contexts in which github issue urls are accepted.