allow content in edit options like in message send options (#1657)

src/structures/Message.js

@@ -359,6 +359,7 @@ class Message {
   /**
    * Options that can be passed into editMessage.
    * @typedef {Object} MessageEditOptions
+   * @property {string} [content] Content to be edited
    * @property {Object} [embed] An embed to be added/edited
    * @property {string|boolean} [code] Language for optional codeblock formatting to apply
    */
@@ -381,6 +382,7 @@ class Message {
     } else if (!options) {
       options = {};
     }
+    if (typeof options.content !== 'undefined') content = options.content;
 
     if (typeof content !== 'undefined') content = Util.resolveString(content);