Feature - programmatically modify toolbar contents

* setContent - clear and set the toolbar items
* addContent - append a toolbar item to the end (right-most side)
* emptyContent - remove all items from the toolbar
Demo - simplify the demo start process with an npm start script
Break out keytable to `lib/keytable.js`
Use 0-based index for switching toolbars

* `switchToolbar` should expect a zero-based index of button to select
* `onKeyDown` should call `switchToolbar` with an index
0.1.0