optional `off` argument to off a specific handler

[jcppman]

Hi, thanks for this great bridge!

In some scenario, on js side an additional optional second argument of off to only unsubscribe specific handler could be really helpful.

function firstHandler(){
  console.log('I am on fire');
}
function secondHandler(){
  console.log('me too');
}

Jockey.on('myEvent', firstHandler);
Jockey.on('myEvent', secondHandler);
Jockey.off('myEvent', secondHandler);

// when event 'myEvent' comes:
// output is only 'I am on fire'

I'd happy to provide a PR if other ppl also like to have this feature