text: Title: Feature request: browse GitHub repo branches
  Body: Currently it's possible to type in a GitHub username and browse their repositories. It's then possible to browse a repository to look for notebooks.
  
  In some cases I'd like to be able to access a notebook that is in a branch other than master. For instance, in many cases it would be particularly useful to access the gh-pages branch of a repository that stores a GitHub Pages site.
  
  So far as I can see there's no UI to access the branches of a repository, though it is possible to access a branch by manually altering the URL. It would be nice to be able to browse branches.
  