Add support for once

[fregante]

• Closes Add support for {once: true} #30

cc @stof

Matches the native once behavior:

document.addEventListener('click', console.log, {once: true})
document.addEventListener('click', console.log)
// The second one is ignored, the listener is called once

document.addEventListener('click', console.log)
document.addEventListener('click', console.log, {once: true})
// The second one is ignored, the listener is called without limits

[fregante]

I think this currently doesn't properly support arrays of elements, like:

delegate([a, div], 'span', 'click', console.log, {once: true})

But I'll merge it for now and open an issue, it's a rare usage and probably should be dropped altogether.