Allow setting attributes when building elements

Add _attributes_ to templates so users can define custom attributes to
set on the processed element

Example from my own code:

<!-- build:style inline small [media="screen and (max-width: 480px)"]
-->
<link rel="stylesheet" media="screen and (max-width: 480px)"
type="text/css" href="css/small.css" />
<!-- /build -->

Creates a element along the lines of
<style media="screen and (max-width: 480px)">/*stuff*/</style>