add User#dmChannel (#1005)

* add User#DMChannel

* Update User.js

* Update User.js

src/structures/User.js

@@ -164,6 +164,14 @@ class User {
     return channel._typing.has(this.id) ? channel._typing.get(this.id).elapsedTime : -1;
   }
 
+  /**
+   * The DM between the client's user and this user
+   * @type {?DMChannel}
+   */
+  get dmChannel() {
+    return this.client.channels.filter(c => c.type === 'dm').find(c => c.recipient.id === this.id);
+  }
+
   /**
    * Deletes a DM channel (if one exists) between the client and the user. Resolves with the channel if successful.
    * @returns {Promise<DMChannel>}