A way to opt out of traits for a type

[andrew-otiv]

One of our types, an enum containing alarm severities, sorts backwards with the default Ord impl. We would like to provide a correct impl for that one type, but we can't since it's already defined by prost.

It would be useful to have a mechanism for opting out of one or more of the default trait implementations for a specific type.

Related to #86 and the PR https://github.com/tokio-rs/prost/pull/96/files

Thanks!