dub check

[Nordlöw via Digitalmars-d]

Rust 1.16 just introduced crate-level compiler checking via

     cargo check

as described at 
https://blog.rust-lang.org/2017/03/16/Rust-1.16.html

Could we extend DUB in a similar way to provide `dub check` that 
simply calls the compiler with the flag `-o-`?