Support item css links.
Support item css links.

If you want to specify some CSS files for a specific item, then you can
add snippet like:
```
css:
  - /static/stylesheets/some-css.min.css
```
to the metadata file(yaml file) of that item.

nanoc-plate/nanoc-plate,nanoc-plate/nanoc-plate,nanoc-plate/nanoc-plate,nanoc-plate/nanoc-plate