Make the EventLoop support External Runtime

[Shnitzelil]

The previous implementation creates New Runtime.
If you already have one you may now use it.

[dop251]

The current design is deliberate (see #91). To add to that, if you need to run initialisation code to set up the *Runtime, you can do that using loop.RunOnLoop(), i.e.

	loop := NewEventLoop()
	loop.Start()
	defer loop.Stop()

	ch := make(chan error)
	loop.RunOnLoop(func(r *goja.Runtime) {
		r.SetMaxCallStackSize(1000)
		// ... More initialisation
		ch <- nil
	})
	err := <-ch

	if err != nil {
		// Handle error
	}

[Shnitzelil]

Thank you for your input.
This is a kind of workaround.
Please consider my idea.

Thank you