Allow any attributes on <template>

[zcorpan]

From #541

<template class="js_globalmessage"> <div class="globalmessage {{type}}"> <div class="messagecontainer"> {{message}} {{#if closable}} <button type="button" class="close js_close">&#x2a09;</button> {{/if}} </div> </div> </template>    

Hmm... Having {{#if foo}} etc as text makes it hard to check content models, e.g. in an ul... And this won't work inside e.g. tables. Maybe it would be better to use nested template elements?:

<template class="js_globalmessage">
 <div class="globalmessage {{type}}">
  <div class="messagecontainer">
   {{message}} 
   <template data-if="closable">
    <button type="button" class="close js_close">&#x2a09;</button>
   </template>
  </div>
 </div>
</template>

(Maybe the template element should allow any attribute, like embed, so you can do if="..." etc?)