Add "once" event hints.

[davidlehn]

Some events are known to be only sent once such as startup, ready, setup, and configuration events. When these are emitted it would be good to add a hint that these are only sent once. Code could then output a warning if there is a listener that is not using once(). This would help clean up some listener resources that are not needed for the full app life.

An API might be to use an object for emit. Something like:

events.emit({name: 'myevent', once: true}, ...args)