Add IPField interface to support IP address retrieval (#60)

- Add an interface for IP Field

document.go

@@ -14,7 +14,10 @@
 
 package index
 
-import "time"
+import (
+	"net"
+	"time"
+)
 
 type Document interface {
 	ID() string
@@ -83,6 +86,10 @@ type GeoShapeField interface {
 	GeoShape() (GeoJSON, error)
 }
 
+type IPField interface {
+	IP() (net.IP, error)
+}
+
 // TokenizableSpatialField is an optional interface for fields that
 // supports pluggable custom hierarchial spatial token generation.
 type TokenizableSpatialField interface {