Write autocomplete file to OUT_DIR
Write autocomplete file to OUT_DIR

This is per the `clap` documentation at
https://docs.rs/clap/2.32.0/clap/struct.App.html#method.gen_completions.
Otherwise, users who pull in `xxd-rs` as a library dependency end up having
`_xxd-rs` files written out in their own project's root.

Nicoretti/xxd-rs