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>

cmd/containerd/builtins.go

@@ -2,7 +2,7 @@ package main
 
 // register containerd builtins here
 import (
-	_ "github.com/containerd/containerd/differ"
+	_ "github.com/containerd/containerd/diff/walking"
 	_ "github.com/containerd/containerd/services/containers"
 	_ "github.com/containerd/containerd/services/content"
 	_ "github.com/containerd/containerd/services/diff"