Step counter feature

[Grimones]

Hi

This PR implements step counter for <Appear /> in <Slide />
To use simply add getAppearStep prop to <Slide /> which is a callback and returns steps.

<Slide ... getAppearStep={ steps => /* do something */ } />

Also added a slide to example.

Based on this branch https://github.com/pedrottimark/spectacle/tree/step-visitor

Use case can be like mine. I'm writing a presentation about ThreeJS and i have a canvas with renderer on slide background. On each step of text appearing i want to do something in ThreeJS.
I have a component with ThreeJS scene
<CubeDemo step={this.state.cubeDemoStep} />
And on each step i do something different in scene

animate() {
    const delta = this.clock.getDelta();
    if (this.step === 1) {
      this.mesh.rotation.x += delta;
    }
    else if (this.step === 2) {
      this.mesh.rotation.z += delta;
    }
    else if (this.step === 3) {
      this.mesh.rotation.x += delta;
      this.mesh.rotation.z += delta;
    }
}

[kenwheeler]

👍