Usage line for subcommands should include bin_name prefix

[joshtriplett]

When showing the usage for a subcommand, clap only shows the subcommand name, not the command name before it. For instance, given a command command, and a subcommand sub, command help sub will show:

sub
Subcommand about string goes here.

USAGE:
    sub [FLAGS] <arg>...

This should look like:

command sub
Subcommand about string goes here.

USAGE:
    command sub [FLAGS] <arg>...

[kbknapp]

This is also a bug, and I'm curious where this regressed. I should look like:

command-sub
Subcommand about string goes here.

USAGE:
    command sub [FLAGS] <arg>...

[joshtriplett]

Why command-sub rather than command sub?

[kbknapp]

I think originally I modeled it after git where all subcommands in manpages have a hyphen separating commands.

[kbknapp]

I have a local branch with this fixed. I'm going to roll it up with a few of the other issues and push the PR.