Derive structopt::StructOpt in Command!

As a preparation for the structopt transition, we derive StructOpt for
the enums generated by Command! so that they can be used as a
subcommand.

src/arg_util.rs

@@ -80,7 +80,7 @@ macro_rules! Command {
  impl_default!($name => $( $name::$var(::std::default::Default::default()) , )*);
  };
  ( $name:ident, [ $( $var:ident => ($str:expr, $exec:expr), ) *] ) => {
- #[derive(Debug, PartialEq)]
+ #[derive(Debug, PartialEq, ::structopt::StructOpt)]
  pub enum $name {
  $(
  $var,