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.
I mean, it is a dependency on another script in this repo, where there is already an assumption ([documented in the `README`](https://github.com/ryan-williams/git-helpers/blob/master/README.md#setup), [implemented in the repo's setup script](https://github.com/ryan-williams/git-helpers/blob/master/.git-rc#L11), and [used by other scripts in this repo](https://github.com/ryan-williams/git-helpers/search?utf8=%E2%9C%93&q=%22from+util%22&type=Code)) that `$PATH` and `$PYTHONPATH` will be such that scripts in this repo can call other scripts in this repo, particularly those in `$repo/util/`.

The use case of cherry-picking some scripts from this repo into your `~/bin` is interesting, though, so we can discuss how to better enable that flow, for users who want some but not all of these scripts.

We could modify the repo's installation assumptions (currently: `source $repo/.git-rc`) to be finer-grained, e.g.:
* mandatory: add `$repo/util` to `$PYTHONPATH` (`util/` containing things that may be reused between scripts)
* optional: add `$repo/` and `$repo/aliases` to `$PATH`, if you want *everything* to be on your path.
    * otherwise, simply `ln -s "$repo/foo" "~/bin/foo"` the things that you want, trusting that they will be able to find their dependencies thanks to the mandatory first bullet above.