Add thing-at-point support for several `github-*' symbols

[vermiculus]

This should make it possible for more packages to work with Magithub without really 'knowing' about it.

• Remove function magithub-thing-at-point and replace with a private implementation of the same logic.

• Defines several thingatpt-enabled symbols:

  • github-user
  • github-issue
  • github-label
  • github-comment
  • github-repository
  • github-pull-request
  • github-notification

  Each of these symbols uses the implementation of magithub-thing-at-point (now called magithub--section-value-at-point) to get the relevant section data.

[vermiculus]

@tarsius Is this functionality something you'd be interested in using in Magit? It seems like the most emacs-y solution to integrating with Magithub without actually depending on it.

I can work on a PR if so; if you have any suggestions for behavior (e.g., whether it should only ask if the relevant thing-at-point is nil, etc.), that'd be helpful 😄

Edit: I've created the below PR on Magit to discuss; this conversation probably makes more sense over there. (The conversation will likely impact what's done here, though.)