Reload new stylesheets in-place.

Add a new optional parameter to resource.create, the URL of the resource to reload.
When creating a new stylesheet and this param is set, add the new element at the same position as the existing one.
Will you add a comment above this line explaining that if `prevUrl` is defined, it will be used for in-place replacement of stylesheets and that this works because previous elements aren't destroyed until after they're loaded?