Currently it just prints the help message and exits 0.
We do this by modifying the helpCommand and helpSubcommand cli.Commands
to return an error if they are called with an unknown subcommand. This
propogates up to the app which exits with 3 and prints the error.
Thanks to @danslimmon for the initial approach!
Fixes#276
plus breaking out "setup" portion of `App.Run` into its own method, cleaning up
some bits of the help templates, and allowing for runtime opt-in of displaying
template errors to stderr.
