Make this install as a git subcommand

[christopherhein]

With the git binary you can have other git subcommands by naming them git-{somename} it would be nice to make this feel like a native command like they did with hub but without requiring aliasing.

It would be nice to install this into your path automatically at $PATH/git-issue-viewer or similar.

This would allow out of the box the user to use git issue-viewer to list then operations that include more subcommands just gett added to the end like git issue-viewer 1 to view the first issue.