Add Message.edits (#636)

* Add Message.edits

* Make travis happy

* Change @type for consistency

src/structures/Message.js

@@ -123,6 +123,8 @@ class Message {
       }
     }
 
+    this._edits = [];
+
     /**
      * Whether or not this message was sent by Discord, not actually a user (e.g. pin notifications)
      * @type {boolean}
@@ -147,6 +149,15 @@ class Message {
     return new Date(this._editedTimestamp);
   }
 
+  /**
+   * An array of cached versions of the message, including the current version.
+   * Sorted from latest (first) to oldest (last).
+   * @type {Message[]}
+   */
+  get edits() {
+    return this._edits.slice().unshift(this);
+  }
+
   patch(data) { // eslint-disable-line complexity
     if (data.author) {
       this.author = this.client.users.get(data.author.id);