Feature Request: Enable reusable workpool for non-ffi use

https://github.com/dropbox/rust-brotli/blob/master/c/README.md#multithreaded-compression

This doc states that it's possible to use the C FFI to create a workpool that can be reused, but looking at the multithreading module in the safe rust crate, I see that the option to use bring your own workpool is missing and so it spawns new threads per `compress_multi`. It would be really nice to be able to do this, and seems like a logical next step since using the C FFI isn't all that friendly for the average rust user who may want to just use `rayon` or some other thread pooling crate for this.