Feature request: Return `callback` function on methods.

## Feature request: Return `callback` function on methods.

If we turn off a listener, it requires `type, selector, callback` right? In this case, we write same function again and again.

For example:

```js
events.on("click", "document", function(){
return "Hello!";
});

...

events.off("click", "document", function(){
return "Hello!";
});
```

If the `on()` method give us callback function, we use it again and again.

Example:

```js
callback = events.on("click", "document", function(){
return "Hello!";
});

...

events.off("click", "document", callback);
```

In this case, you just add `return callback;` end of methods.

Start by locating the on() and off() method implementations in the events library and check how listener callbacks are stored and matched. Confirm the returned value can be passed to off() for the same event and selector, then run the existing test suite or add coverage if the project provides tests.

Written by the indexing model from the issue text.