I've been thinking about adding @fizgen to the reviewers in servo/rust-bindgen.

I think he can perfectly get access to full reviewer privileges, but there are
also a few contributors that I've been considering giving access to, and those
don't need full Servo reviewer access.

This adds a way to add per-repo reviewers to the travis repos, something I guess
it's also useful for rust-url, where there are more people with write
permissions.

The way this is organized makes easier to add "out-of-the-org" repos too.