Support --jobs on publish argument for build parallelism

[adamcrume]

cargo build supports a --jobs argument for controlling the build parallelism, and this is very useful for controlling the parallelism of non-Rust builds for *-sys crates. cargo publish should support the same option.

[alexcrichton]

Ah good point! This should be relatively easy to do, so I'm tagging with E-easy. This largely just needs to modify src/bin/publish.rs and then plumb the -j option all the way down to where the crate is actually verified. Feel free to ask questions if someone needs help implementing this!

[mattscamp]

I will take it. Just to confirm @alexcrichton, we want to pipe to here?

[alexcrichton]

Indeed! (although it looks like you've already done it)

[brson]

Fixed. Thanks @mattscamp !