Allow usage of directives as element attributes

The Chrome inspector (for example) does not handle unrecognized HTML
elements well.

Where one would expect the directive elements to expand and encompass
child elements (children being the svg element), the inspector shows
a height and width of NaN and the elements appear to be collapsed
when inspecting them.

To make it easier to see how the directive elements are affecting the page
flow, this pull request enables them to be used at attributes.

Thanks for considering! And thanks for making this awesome project. It has
already saved me countless hours.

Cheers,

-Jedidiah

![screen shot 2013-10-30 at 8 48 03 am](https://f.cloud.github.com/assets/862838/1437737/53cc7b6c-4172-11e3-904b-d6d334a318a7.png)