encoder: allow encoders for top-level proto?

[mping-exo]

Related to #22.

I think it would be useful to allow encoders for top-level protobuf objects:

(defmapper mapper [ObjectReference]
           :encoders {ObjectReference
                      {:from-proto (fn [^ObjectReference proto-obj] ...)
                       :to-proto   (fn [clj-map] ...)}})

This would be useful for cases where we want to use a clojure type that has no native representation in protobuf, such as keywords or UUIDs.
Besides, it would make the library more orthogonal, since it already supports mappings for nested types.