Improve logic for initialising reading node tiddlers

We want fine control of whether tiddlers are read from the DOM or the
file system, without ganging it to whether we’re on node vs. browser.
Move node-webkit saver into a plugin

We don’t need it for TiddlyDesktop now that it reuses the TiddlyFox
saver. But it is still useful for embedding a TW directly into a
node-webkit app
Add link to French thesis notebook