self-references

[Rich-Harris]

Ref #51. This PR adds self-references, so that a component can do this sort of thing:

<article class='file {{file.type}}'>
  {{file.name}}

  {{#if file.type === 'folder'}}
    <ul>
      {{#each file.children as child}}
        <li><:Self file='{{child}}'/></li>
      {{/each}}
    </ul>
  {{/if}}
</article>

I went with :Self because it doesn't interfere with syntax highlighting (at least in the places I've looked), and using a special name over other mechanisms (like late-binding components, or using a special keyword like this inside the components object) means that we can easily guard against infinite recursion by ensuring that it's only used inside if or each blocks.