add override saver (#4908)

* add override saver

* rename 'override' to 'custom' and use the  global instead of introducing a new one

core/modules/savers/custom.js

@@ -0,0 +1,60 @@
+/*\
+title: $:/core/modules/savers/custom.js
+type: application/javascript
+module-type: saver
+
+Looks for `window.$tw.customSaver` first on the current window, then
+on the parent window (of an iframe). If present, the saver must define
+	save: function(text,method,callback) { ... }
+and the saver may define
+	priority: number
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var findSaver = function(window) {
+	try {
+		return window && window.$tw && window.$tw.customSaver;
+	} catch (err) {
+		// Catching the exception is the most reliable way to detect cross-origin iframe errors.
+		// For example, instead of saying that `window.parent.$tw` is undefined, Firefox will throw
+		//   Uncaught DOMException: Permission denied to access property "$tw" on cross-origin object
+		console.log({ msg: "custom saver is disabled", reason: err });
+		return null;
+	}
+}
+var saver = findSaver(window) || findSaver(window.parent) || {};
+
+var CustomSaver = function(wiki) {
+};
+
+CustomSaver.prototype.save = function(text,method,callback) {
+	return saver.save(text, method, callback);
+};
+
+/*
+Information about this saver
+*/
+CustomSaver.prototype.info = {
+	name: "custom",
+	priority: saver.priority || 4000,
+	capabilities: ["save","autosave"]
+};
+
+/*
+Static method that returns true if this saver is capable of working
+*/
+exports.canSave = function(wiki) {
+	return !!(saver.save);
+};
+
+/*
+Create an instance of this saver
+*/
+exports.create = function(wiki) {
+	return new CustomSaver(wiki);
+};
+})();

licenses/cla-individual.md

@@ -415,3 +415,5 @@ Kamal Habash, @Kamal-Habash, 2020/08/28
 Florian Kohrt, @fkohrt, 2020/09/10
 
 Gerald Liu, @gera2ld, 2020/09/25
+
+Ryan Kramer, @default-kramer, 2020/10/24