Example for an external subcommand.

[marmistrz]

Add an example for spawning an external subcommand with a multiply existing argument. This was pretty hard for me to find.

Why is the subcommand not displayed in help? I get:

app [OPTIONS]

I'd expect

app [OPTIONS] [COMMAND]

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This change is 

[coveralls]

Coverage remained the same at 91.929% when pulling ee2f5ec on marmistrz:master into 76869a0 on kbknapp:master.

[kbknapp]

I think it'd be good to add another statement down below the println showing how to use a match and pull those "additional subcommand" args out of the grep external subcommand.

[marmistrz]

I don't really understand what you mean. I think I could alternatively just launch the subcommand - this sounds like a real-life example.

Anyway, this can't be merged until #1093 is resolved.

[kbknapp]

@marmistrz sorry, I realize that wasn't super clear of me. I mean how to send all the options and such to the external subcommand, like in this example