Feature/pure components

[GianlucaGuarini]

Add pure components rendering

[GianlucaGuarini]

This PR enables the use of riot.pure to handle the riot components rendering however you like. For example if you want to use lit-html as rendering engine you can use it directly in your components:

<lit-element>
<script>
  import { pure } from 'riot'
  import { html, render } from 'lit-html'

  export default pure(({ attributes, slots, props }) => ({
    mount(el, context) { 
      this.el = el
      this.render(context)
    },
    // context here is either the parent component or undefined
    render(context) {
      render(html`<p>{ context ? context.message : 'no message defined' }</p>`, this.el)
    },
    unmount() {
      this.el.parentNode.removeChild(this.el)
    }
  }))
</script>
</lit-element>