Add current named step on step change callback

[ayresflesch]

I was wondering if there's a way to add the active named step to the stats object used on step change callback.

react-step-wizard/src/index.js

Lines 114 to 123 in c2b9448

	this.setState({
	activeStep: next,
	classes,
	}, () => {
	// Step change callback
	this.onStepChange({
	previousStep: active + 1,
	activeStep: next + 1,
	});
	});

Something like this, I guess:

// Step change callback
this.onStepChange({
    previousStep: active + 1,
    activeStep: next + 1,
    activeNamedStep: // get named step
});