Local post assets with Jekyll

I wrote a plugin to let me organise assets in subdirectories easily:
https://github.com/samrayner/jekyll-asset-path-plugin

{% asset_path my-image.png %}

in post 2013-01-01-post-title would output:

/assets/posts/post-title/my-image.png

in page my-first-page would output:

/assets/my-first-page/my-image.png