Initial work to try and get gists to update. Not finished yet.
Implement updating gists.

BOO YAH! OPFUNC IS LITERALLY MAGIC!!! <3 Vim
Restrict updating gists when `g:fist_anonymously` is false.
Implement updating gists. [Merge branch 'update-gist']

* update-gist:
  Restrict updating gists when `g:fist_anonymously` is false.
  Implement updating gists.
  Initial work to try and get gists to update. Not finished yet.