Makes listening to multiple events on one target slightly easier.
Adding events can be rolled up into passing through an object, and the
returned remove function removes all of the handlers in the object
For example:
```
//this.listen...
const remove = listeners.add(
{
'message': handler,
'error': handler
}
);
remove(); // removes all listeners in the object
```
The entire API for listeners is now becoming slightly overloaded, so
potentially we'd use this API always and remove the ability to use a
string/function pair.
011fb8a559