function block(name, html) {
  // bound to the blocks object in renderFile
  var blk = this[name];
  if (!blk) {
    // always create, so if we request a
    // non-existent block we'll get a new one
    blk = this[name] = new Block();
  }
  if (html) {
    blk.append(html);
  }
  return blk;
}
Return the block with the given name, create it if necessary.
Optionally append the given html to the block.

The returned Block can append, prepend or replace the block,
as well as render it when included in a parent template.

@param  {String} name
@param  {String} html
@return {Block}
@api private