Feature request: subgraph as a node

Steps to reproduce

digraph G {
   subgraph cluster_0 {
      a;
      b;
   }
   cluster_0 -> abc;
}

Expected Behaviour

I hope there will be an edge starting from cluster_0 to abc.

Actual Behaviour

The syntax above is not supported.

OS Version

General syntax.

Graphviz Version

This is a feature request.

Additional info

Graphviz is the de facto standard on dot language. If a subgraph can inherit all the features of a node, the graph can scale a lot. Please consider this feature request! Great thanks!