Causes the wiki to be autosaved whenever clicking “done” after editing
a tiddler. Only works with savers that support autosave. We should
probably make autosave configurable
We were re-using the `tw-save-wiki` message both for saving the current
wiki and downloading a new wiki. Now we’ll use the separate
`tw-download-file` message for downloading.
Fixes#236
It's not great, sadly. Clicking save in the browser pulls up an
unobtrusive bar at the bottom of the browser window where you can click
"save". You then get a new copy of your wiki in the downloads folder.
