Allow setting attributes when building elements

When using your builder I had the need to set the `@media` attribute on some compiled elements. It was easy enough to add to your syntax an additional attributes property of the format `[attributes='last item wrapped in square brackets']`.

Add _attributes_ to templates so users can define custom attributes to
set on the processed element

Example from my own code:

```html
<!-- 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

```html
<style media="screen and (max-width: 480px)">/*stuff*/</style>
```

Issues:

- The regexp I implemented isn't greedy so you cannot have nested brackets in the attributes. Cant think of a single instance where this will be a problem for the supported html elements.