Add a GitHub saver

Fixes #3890

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

editions/prerelease/tiddlers/Release 5.1.20.tid

@@ -15,6 +15,8 @@ Added serveral new string operators: [[length|length Operator]], [[uppercase|upp
 
 Added several new palettes. See the [[palette manager|$:/core/ui/ControlPanel/Palette]].
 
+Added new [[GitHub saver|Saving to GitHub]].
+
 !! Plugin Improvements
 
 New and improved plugins:

editions/tw5.com/tiddlers/definitions/GitHub.tid

@@ -1,5 +1,5 @@
 created: 20140910212609354
-modified: 20140910212725412
+modified: 20190408173002622
 tags: Definitions
 title: GitHub
 type: text/vnd.tiddlywiki
@@ -8,4 +8,6 @@ GitHub is a hosting service for distributed projects that use git as their versi
 
 The code and documentation of TiddlyWiki is hosted on GitHub at:
 
-https://github.com/Jermolene/TiddlyWiki5
+https://github.com/Jermolene/TiddlyWiki5
+
+GitHub also offer a free web hosting service called [[GitHub Pages|https://pages.github.com/]] that can be used directly from the single file configuration. See [[Saving to GitHub]].

editions/tw5.com/tiddlers/saving/Saving to GitHub.tid

@@ -0,0 +1,24 @@
+created: 20190408173002622
+modified: 20190408173002622
+tags: Saving Android Chrome Firefox InternetExplorer iOS Linux Mac Opera Safari Windows
+title: Saving to GitHub
+type: text/vnd.tiddlywiki
+delivery: Service
+method: save
+caption: GitHub Saver
+description: Save changes directly to a GitHub repository
+
+TiddlyWiki can save changes directly to a GitHub repository in the single file configuration.
+
+Saving to GitHub is configured in the [[$:/ControlPanel]] in the ''~GitHub Saver'' tab under the ''Saving'' tab. The following settings are supported:
+
+* ''Username'' - (mandatory) the username for the GitHub account used for saving changes
+* ''Password'' - (mandatory) the password, OAUTH token or personal access token for the specified account. Note that GitHub permits [[several different mechanisms|https://developer.github.com/v3/#authentication]] for authentication
+* ''Repository'' - (mandatory) the name of the GitHub repository. Both the owner name and the repository name must be specified. For example `Jermolene/TiddlyWiki5`
+* ''Branch'' - (optional) the name of the branch to be used within the GitHub repository. Defaults to `master`
+* ''Path'' - (optional) the path to the target file. Defaults to `/`
+* ''Filename'' - (mandatory) the filename of the target file
+
+Notes
+
+* The GitHub password is stored persistently in browser local storage. Be sure to clear the password if using a shared machine. Using a [[personal access token|]] for authentication offers an extra layer of security: if the access token is accidentally exposed it can be revoked without needing to reset the account password