Adding system-co-using for adding explicit co-dependencies

Sometimes you need to add a dependency (or co-dependency) on an existing
component. For example, you may want to contribute a new set of bidi
routes to a router which has already been defined. This function allows
you to do that in the same way as component's system-using.
Merge pull request #1 from malcolmsparks/master

Adding system-co-using for adding explicit co-dependencies