ZSH normal completion for args in "cargo run -- <args>"

Running a Rust program with some arguments such as files is a common use
case. The ZSH completion did not declare support for extra arguments to
"cargo run", so file completion was impossible. In practice pressing TAB
after writing "cargo run -- " gave no completion at all.

After this patch, cargo run can do "normal completion",
which in practice does file/directory completion. This works nicely
with "cargo run -- ".