fix(Help): prevents invoking <cmd> help help and displaying incorrect help message
fix(Help): prevents invoking <cmd> help help and displaying incorrect help message

Previously, if one ran `<cmd> help help` an additional `help` subcommand was created and a help
message displayed for it. We *could* have just thrown an error `<cmd> help help` but I worry that
the message would be confusing, because something like, "Invalid Subcommand" isn't 100% correct as
the form `<cmd> help <subcmd>` is allowed, and there *is* a `help` subcmd.

This fix correct dispatches `<cmd> help help` to the `<cmd>` help message.

Closes #538

clap-rs/clap,kbknapp/clap-rs,kbknapp/clap-rs,clap-rs/clap,clap-rs/clap