Add support for clearlydefined.io

[Jake-Shadle]

Clearlydefined.io has an api that can be used to get license information for crates, which would be a more robust addition to the current checking we do. It could also be accelerated by caching results that can be checked in so that requests don't need to be made for the same crate and version.

[Evrey]

As long as querying some random web service is an opt-in feature I can ignore, I am okay with this.