Compare command

I heart the compare view, but I have a hell of a time both finding it in the UI and remembering the exact format of the URL. Therefore, it'd be nice to have a command to make it a non-issue to remember:

    gh compare my-awesome-sauce                     # => https://github.com/user/repo/compare/master...my-awesome-sauce
    gh compare lame-sauce my-awesome-sauce   # => https://github.com/user/repo/compare/lame-sauce...my-awesome-sauce

These only print the URL. If you want to open it in a browser, using launchy, you just toss on an `--open`:

    gh compare my-awesome-sauce --open

Right now, it's pretty naive in how it generates the compare URL. That is, the args to `gh` are put directly in the URL. I have some crazy ideas brewing though where you would be able to pass it treeish objects in your local repos (ie branch quantified by remote), and it'd do the trickery to convert that to the URL syntax. For example, if I had a remote called upstream that pointed at the trogdor user, you might do this:

    gh compare upstream/my-awesome-sauce                     # => https://github.com/user/repo/compare/master...trogdor:my-awesome-sauce

Not quite there yet though, so definitely open to feedback!