Support help for commands and sub-commands

[cbracken]

Today, dvm help only ever displays the top-level usage, never anything command or sub-command specific:

% dvm help alias create
usage: dvm <command> [<args>]
...

dvm does actually have useful help strings, but users need to enter an invalid command to get them:

% dvm alias
usage: dvm alias <create|update|delete|list> [<args>]

% dvm alias create
usage: dvm alias create <alias> [--path] <version>