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.
If you feel strongly about a workflow where scripts like this are just `wget`ed directly into `~/bin` and nothing else happens then let's discuss more.

One example I have is that I was already using some git autocomplete libraries that requires `source`ing (that are now [incorporated into this repo](https://github.com/ryan-williams/git-helpers/blob/master/.git-rc#L7)). In general that pattern seems reasonable to me.

Also, if your ideal flow is `wget http://.../foo-script -P ~/bin/`, then I would argue it's not that much worse if it instead had to look like `wget http://.../install-foo -P /tmp && /tmp/install-foo`, at which point we can do whatever we want.

Final thought: in the spirit of the `issues` command, which I am very excited for, I could imagine adding many more cmdline github helpers: `watch`, `unwatch`, etc., involving github API interactions. These might take shorthands for github URLs, e.g. any of `https://github.com/ryan-williams/git-helpers`, `ryan-williams/git-helpers`, or just parse the github URL from `git remote -v`, like here. Stuff like this would be great plumbing to reuse across many scripts.

tl;dr, I guess, is that I do see this more as a potential framework than a collection of one-off/entirely-self-contained scripts.