* letsencrypt --help $SUBCOMMAND now works.
Fixes#787#819
* subcommand arguments are now actually argument groups, so that all flags
can be placed before or after subcommand verbs as the user wishes
Fixes: #820
A limitation:
* args like --cert-path were previously present for multiple verbs
(auth/install/revoke) with separate docs; they are now in the "paths"
topic. That's fine, though it would be good to *also* list them when the
user types letsencrypt --help install.
2e0fd36c28