New binary: 'issues' to easily manage large volumes of github issues.

* `issues list --assigned '*'` lists all open *assigned* issues.
* `issues list --assigned 'none'` lists all open *unassigned* issues.
* `issues list` lists all open issues, assigned or not (default).
* `issues alru` prints the Age of the Least Recently Updated issue (ALRU). Lower is better.
Merge pull request #12 from shykes/issues

New subcommand: 'issues' lists all open issues, with optional filtering by assignee