Add issues command, which opens github issues for a repo

Just came up with this command, I think it's gonna be great!

While you're in a git repo on your command line, you run `issues` and it opens the github issues for that repo in your browser.
since we're in this repo, use [`remotes.py`](https://github.com/ryan-williams/git-helpers/blob/master/util/remotes.py)?

    from util.remotes import get_remotes

    [ open_issues_url(remote.group('path')) 
        for remote in get_remotes().values() 
        if remote.group('host') == 'github.com' ]

:smile: