Support setting attributes without directives using @.

Previously to set attributes besides the default (usually text contents,
sometimes checkbox or `option` inside a `select`) you needed to use
directives. This patch permits the use of the notation
`selector@attribute` to simplify tasks like setting `href` on links or
`src` on images.

Example:

Template:

    <html>
      <div id="words">words words</div>
    </html>

Data:

    {"words": "blah blah", "words@style": "color:red"}

Result of applying template:

    <html>
      <div id="words" style="color:red">blah blah</div>
    </html>

-POLM