Add route for manifest

Also allows any :version param in theme urls
(but will still always serve the current
working version)
Add .iago/config.json file to hold configuration
Adds start script

After a local `npm install iago`

you can now `npm start iago` to start
the server
Theme preview works in source or via npm install

That is, we now think of the "root path" for
themes as the path into which iago is
"npm install"ed. If none, it's the current
working directory (e.g., when running `npm start`
just from the iago repo itself).