Support for listing all (open & closed) issues #191

This supports the listing all of issues on a repo, by use of a `--state all` command line option. This doesn't color the output because I actually couldn't figure out how to use the `format_issues_header` method, as it seems to do some weird stuff with the output.

As such, this is a non-fancy add on that prints `# Open/Closed issue` before listing each group.

Example usage. `ghi list --state all`