fix(Util::asset_url): allow use without passing a current_resource
fix(Util::asset_url): allow use without passing a current_resource

Most cases the current_resource isn’t actually needed, so calls could be made
from extensions and still get the correct path back.

An exception will be raised with a descriptive message if the current_resource
is needed but not given. (for example, if relative is true — it needs a resource
to base the url on)
middleman/middleman,mvastola/middleman,mvastola/middleman,puyo/middleman,mojavelinux/middleman,mvastola/middleman,mojavelinux/middleman,middleman/middleman,mojavelinux/middleman,mojavelinux/middleman,mvastola/middleman,middleman/middleman,middleman/middleman,puyo/middleman,puyo/middleman,puyo/middleman