*Add peerHasItem method

src/index.ts

@@ -89,6 +89,13 @@ export default class DHTCache extends EventEmitter {
     return items.sort();
   }
 
+  public peerHasItem(peer: string | Buffer, item: string | Buffer): boolean {
+    peer = this._maybeHexify(peer);
+    item = this._maybeHexify(item);
+
+    return this.graph.hasSuccessor(peer, item);
+  }
+
   public addItem(item: string | Buffer) {
     item = this._maybeHexify(item);
     this._cache.add(item);