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>

plugin/differ.go

@@ -1,13 +0,0 @@
-package plugin
-
-import (
-	"github.com/containerd/containerd/mount"
-	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
-	"golang.org/x/net/context"
-)
-
-// Differ allows the apply and creation of filesystem diffs between mounts
-type Differ interface {
-	Apply(ctx context.Context, desc ocispec.Descriptor, mount []mount.Mount) (ocispec.Descriptor, error)
-	DiffMounts(ctx context.Context, lower, upper []mount.Mount, media, ref string) (ocispec.Descriptor, error)
-}