Support for array fields (sub-templates)

See test case #12:
{ title: 'title', chapters: [{ header: 'Preface' }, { header: 'Index'
}] }
<div id=book class=dna-template>
     <div>Title: <span>~~title~~</span></div>
     <div data-dna-array=chapters>Chapter: <span>~~header~~</span></div>
</div>