Request: (Configurable) link-unfurling for GitHub Enterprise links to source-code

[ELLIOTTCABLE]

I'd love to see links of the following forms expanded to inline, language-highlighted source-code:

- https://git.our-github-enterprise.com/anorg/arepo/blob/abranch/path/to/file.ml#L22-L27
- https://git.our-github-enterprise.com/anorg/arepo/commit/99046b102fdb1df2d1bb297328d953faf70d3e3a#diff-5a77bbcd0815bef116f8d229276915e0522f3f271c169a5ffe55b7ebf23a739cR23-R26
- https://git.our-github-enterprise.com/anorg/arepo/pull/11111/files#diff-5a77bbcd0815bef116f8d229276915e0522f3f271c169a5ffe55b7ebf23a739cR28-R32
- https://git.our-github-enterprise.com/anorg/arepo/pull/11111/commits/99046b102fdb1df2d1bb297328d953faf70d3e3a#diff-5a77bbcd0815bef116f8d229276915e0522f3f271c169a5ffe55b7ebf23a739cR28-R32

There was some discussion of this in our Slack; this has to eventually be handled somewhat delicately, as it's possible for this feature to "leak" code that the Slack-poster shouldn't have access to.

At first blush, the best solution is probably a simple repo/channel whitelist; eventually, mapping of Slack-user to GitHub-user may be necessary.

[yasunariw]

thanks! we can always make more link types unfurlable, same with #119

language-highlighted

Slack doesn't support highlighting:-(