Refactor differ into separate package

Add differ options and package with interface.
Update optional values on diff interface to use options.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

api/services/diff/v1/diff.proto

@@ -50,6 +50,10 @@ message DiffRequest {
 	// Ref identifies the pre-commit content store object. This
 	// reference can be used to get the status from the content store.
 	string ref = 4;
+
+	// Labels are the labels to apply to the generated content
+	// on content store commit.
+	map<string, string> labels = 5;
 }
 
 message DiffResponse {