Introduce #[structopt(external_subcommand)]

[CreepySkeleton]

Closes #130

I took some time to develop and debug, but it's finally ready. I have one concern thought.

[CreepySkeleton]

Sorry for the radio silence, the New Year and all :)

This is not abandoned, I'll get to it in 1-2 days.

[CreepySkeleton]

This error message is rather unfortunate: I would very much like it to say "expected OsString", but there's nothing we can do about it 😢

[CreepySkeleton]

This is the best I've managed to come up with:

error[E0308]: mismatched types
  --> $DIR/external_subcommand_wrong_type.rs:13:15
   |
13 |     Other(Vec<CString>)
   |               ^^^^^^^ expected struct `std::ffi::CString`, found struct `std::ffi::OsString`
   |
   = note: expected type `std::vec::Vec<std::ffi::CString>`
              found type `std::vec::Vec<std::ffi::OsString>`

error[E0308]: mismatched types
  --> $DIR/external_subcommand_wrong_type.rs:13:15
   |
13 |     Other(Vec<CString>)
   |               ^^^^^^^
   |               |
   |               expected struct `std::ffi::OsString`, found struct `std::ffi::CString`
   |               this match expression has type `std::vec::Vec<std::ffi::OsString>`
   |
   = note: expected type `std::vec::Vec<std::ffi::OsString>`
              found type `std::vec::Vec<std::ffi::CString>`

While it is indeed possible to draw the error message we desire, I can't get rid of the first one.

[TeXitoi]

That's acceptable

[CreepySkeleton]

OK, ready to go

[CreepySkeleton]

ping @TeXitoi

[TeXitoi]

Great!