[hab] Display help on empty subcommands by default (ex: `hab pkg`).
[hab] Display help on empty subcommands by default (ex: `hab pkg`).

This change adds the default help/usage output when a user types a
subcommand such as `hab pkg`, `hab rumor`, etc. Honestly, a forgotten
"must" :)

habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,nathenharvey/habitat,habitat-sh/habitat