Support zero-mutation hydration via an attribute

[developit]

This extends #23 with a new hydrate generic attribute. When specified, initial render will perform hydration instead of a mutative render:

register(({ color, label }) => (
  <button class="fancy" style={{ padding: 10, color }}>
    {label}
  </button>
), 'fancy-button', ['color', 'text'])

<fancy-button color="red" label="Click Me!">
  <button class="fancy" style="padding:10px; color:red;">Click Me!</button>
</fancy-button>