Typed mimes

This adds type safety to the mimes, check src/mimes.rs in the last commit. [Here is a link](https://github.com/Ryman/nickel.rs/blob/14134540c1d875dca2998bd60716b68af818b52a/src/mimes.rs)

This is currently based on my branch with the finalizers, but I can rebase on master if that doesn't land. Please don't merge until then!