Added an event dispatcher filtering mechanism

[miraclebg]

• Implemented event dispatcher filtering (aka pass a variable so all listeners can alter it and return it back)
• Code formatting

[krasimir]

Hey,

Thanks for the PR. A few things:

• The suggested method looks more like reduce rather then filter. My feeling is thatfilteraccepts a list of items and returns another list of items whilereduce` accumulates a single value.
• Such a method is a little bit out of the scope of the library. For me it sounds like the event listeners need to do an additional work over a variable and because they are part of the EventBus we are adding a new method to perform that work. I'll rather go with the following:

EventBus.addEventListener('my-event', callback1);
EventBus.addEventListener('my-event', callback2);
EventBus.addEventListener('my-event', callback3);
EventBus.addEventListener('my-event', callback4);

var listeners = EventBus.getListeners('my-event');

value = { ... };
var result = listeners.reduce(function (r, listener) {
  // some logic here
  return r;
}, value);

The missing part is getListeners method. So, instead of creating a filter method I'll probably create getListeners that returns all my functions in an array and then use reduce on top of that.

• +1 for the code formatting.
• The new method is not documented in the README

What you think if you add getListeners method and try using it? If it works I'll be happy to merge this PR.