Sightly: allow calling data-sly-use with a resource path

Following the discussion on dev@sling.apache.org I believe it would be nice if we can bind a resource (by path) to a variable using data-sly-use:

data-sly-use.myResource="${ '/content/myResource' }"

This will eliminate the need to create an use object for simple cases when we are reading properties from other resources.