Add support for attributes to r:files

[adamjohnson]

It'd be sweet if hammer would support the attributes for r:files like labels, limit, types, and random. I'd wager the most noticeable ones are limit and maybe random. Code snippet:

<style>
.example {
  background-image: url(<r:files:each labels="random-image-collection" limit="1" types="image" random="true"><r:file:image_url size="1560x636" /></r:files:each>);
}
</style>

Having a look through cleanslate/lib/slate/radius/tags/asset.rb will likely be helpful.

Related to #19.

[zeroedin]

@adamjohnson when you can test out what functionality here is still missing and what needs added.

[zeroedin]

Marking as fixed in 849a79a