New commands: star, unstar, follow, unfollow.

These features are not usable with simple web browsers on github.com.
This is my accessibility workaround.
Adds star, unstar, follow and unfollow as subcommands to hub.
Repo/user info is either taken from the current repo or from the command-line.
See my posting about github accessibility issues
(http://blind.guru/contributing-on-github.html) for my motivation.