Added support for loadCSS and async javascripts

[imathis]

This should fix octopress/asset-pipeline#15

[imathis]

Basically you'll be able to add this to your _config.yml and it will automatically load your CSS asynchronously, but only if it is building the combined file (for production).

asset_pipeline:
  async_css: true

This also sets async javascript to true for combined (production) script tags. Since the order is already defined by the plugins (or author in the case of octopress-asset-pipeline), this accomplishes the same as defer but loads the js in asynchronously.

[imathis]

Here is what it looks like when you generate with async_css: true.

[kogakure]

Nice! All I thought of was to have a path and you include loadCSS and all things needed by default 😮 Thank you for this cool solution! 👍

[imathis]

I started out looking at what it would take to make it work with paths, but the implementation using paths ended up being really inelegant. So I decided that I might as well make it work the way I'd want to use it. Thanks for drawing my attention to this. :)