Create Context Menu (asynchronous) demo doesn't work with right-click

[ericprud]

In documentation/demo/async-create, the button says "right click me" but doesn't behave as expected in:

• firefox - the contextmenu gets rendered after the browser's default context menu has rendered.
• chromium - only the browser's default context menu is rendered.

The async-create demo builds the items immediately upon click event. I think the common use case is that the items take a while to construct and that $this.contextmenu() is fired when they're ready.

Also, the captured event is mouseup, which means it doesn't translate to common cases where folks need mouseup and contextmenu to be different. Switching the event to contextmenu would address this but you theen need to disable the contextmenu handler during execution because the call to $this.contextmenu() triggers it again.

#8 addresses these points with a new demo, which could be moved to async-create.md to remedy the above issues.