Support Multiple Keys in Builders/Parsers

[P.I.E. Security Team]

https://github.com/paseto-standard/paseto-spec/blob/master/docs/02-Implementation-Guide/05-API-UX.md

We should provide the ability to specify an optional Key Ring per Builder / Parser. If the footer contains a Key ID, it will map it to the corresponding key object.

[P.I.E. Security Team]

This was done in #138.

We'll need to document this feature before we tag v2.1.0.