Implement Zoomable block

[molefrog]

Zoomable block allows to zoom a piece of DOM on click. It can be used in order to wrap images and other blocks. We can later connect it to fragments (see #1), but for now zoom on click is fine too.

The possible interface:

import { Zoomable } from 'presa/blocks'

<Slide>
  <Zoomable>
    <img src="..." />
  </Zoomable>
</Slide>