Add support for completion of issue numbers in relevant buffers

[purcell]

This allows completion of #123 issue numbers etc. with vanilla Emacs' completion-at-point mechanism, with extended completion attributes so that company's completion-at-point backend can show further information about candidates.

I was going to publish this as a separate little package, but it arguably belongs in magithub directly. In its current form, there is no change in default behaviour, i.e. completion is never auto-enabled: this is the more conservative choice, but perhaps this functionality warrants loading via the magithub-features mechanism.

Thoughts for future enhancement:

• It'd be nice to complete @user patterns from known user names (relates to discussion of caching users in #262).
• Completing issues in other repos would be useful.
• Patterns such as "PR 123" / "pull request 123" / "issue 123" should perhaps also be completed.

[vermiculus]

This is awesome! Thanks for doing this :smile:

[vermiculus]

Note to myself that this will also close #23.

[purcell]

Okay, added that last bit - please check it over. :-)

[vermiculus]

Looks like there's just a single teeny build bug (just need a declare-function for the compiler, methinks).

Could you add documentation for the completion feature in that variable? :-)

[purcell]

I'll take a look now.

[vermiculus]

Thanks!

[purcell]

Woot, thanks for shepherding this through. I'm happier with this result than if I'd published a separate add-on package. Keep up the good work!

[vermiculus]

It's a great addition 😄