Add a GitHub saver

Fixes #3890

I think it would be useful to have a simple tutorial for setting up saving via GitHub pages.

core/modules/savers/github.js

@@ -0,0 +1,117 @@
+/*\
+title: $:/core/modules/savers/github.js
+type: application/javascript
+module-type: saver
+
+Saves wiki by pushing a commit to the GitHub v3 REST API
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var base64utf8 = require("$:/core/modules/utils/base64-utf8/base64-utf8.module.js");
+
+/*
+Select the appropriate saver module and set it up
+*/
+var GitHubSaver = function(wiki) {
+	this.wiki = wiki;
+};
+
+GitHubSaver.prototype.save = function(text,method,callback) {
+	var self = this,
+		username = this.wiki.getTiddlerText("$:/GitHub/Username"),
+		password = $tw.utils.getPassword("github"),
+		repo = this.wiki.getTiddlerText("$:/GitHub/Repo"),
+		path = this.wiki.getTiddlerText("$:/GitHub/Path"),
+		filename = this.wiki.getTiddlerText("$:/GitHub/Filename"),
+		branch = this.wiki.getTiddlerText("$:/GitHub/Branch") || "master",
+		headers = {
+			"Accept": "application/vnd.github.v3+json",
+			"Content-Type": "application/json;charset=UTF-8",
+			"Authorization": "Basic " + window.btoa(username + ":" + password)
+		};
+	// Make sure the path start and ends with a slash
+	if(path.substring(0,1) !== "/") {
+		path = "/" + path;
+	}
+	if(path.substring(path.length - 1) !== "/") {
+		path = path + "/";
+	}
+	// Compose the base URI
+	var uri = "https://api.github.com/repos/" + repo + "/contents" + path;
+	// Bail if we don't have everything we need
+	if(!username || !password || !repo || !path || !filename) {
+		return false;
+	}
+	// Perform a get request to get the details (inc shas) of files in the same path as our file
+	$tw.utils.httpRequest({
+		url: uri,
+		type: "GET",
+		headers: headers,
+		data: {
+			ref: branch
+		},
+		callback: function(err,getResponseDataJson,xhr) {
+			if(err) {
+				return callback(err);					
+			}
+			var getResponseData = JSON.parse(getResponseDataJson),
+				sha = "";
+			$tw.utils.each(getResponseData,function(details) {
+				if(details.name === filename) {
+					sha = details.sha;
+				}
+			});
+			var data = {
+					message: "Saved by TiddlyWiki",
+					content: base64utf8.base64.encode.call(base64utf8,text),
+					branch: branch,
+					sha: sha
+				};
+			// Perform a PUT request to save the file
+			$tw.utils.httpRequest({
+				url: uri + filename,
+				type: "PUT",
+				headers: headers,
+				data: JSON.stringify(data),
+				callback: function(err,putResponseDataJson,xhr) {
+					if(err) {
+						return callback(err);
+					}
+					var putResponseData = JSON.parse(putResponseDataJson);
+					callback(null);
+				}
+			});
+		}
+	});
+	return true;
+};
+
+/*
+Information about this saver
+*/
+GitHubSaver.prototype.info = {
+	name: "github",
+	priority: 2000,
+	capabilities: ["save", "autosave"]
+};
+
+/*
+Static method that returns true if this saver is capable of working
+*/
+exports.canSave = function(wiki) {
+	return true;
+};
+
+/*
+Create an instance of this saver
+*/
+exports.create = function(wiki) {
+	return new GitHubSaver(wiki);
+};
+
+})();