Support using rustfmt as a lib on stable

I was wondering if rustfmt could be made to work on stable as a library. I would like to depend on it for some code generation (gRPC output), but I cannot require nightly for my library.

There are a [few nightly features](https://github.com/rust-lang-nursery/rustfmt/blob/master/src/lib.rs#L11-L15) being used, but I don't know how critical those are.