Styling subgraph?

It would be nice to be allow to style `subgraph`, the same way we can style nodes, for example like this to change subgraph background color:

```
graph TD
  subgraph A
    B-->C
    B-->D
    C-.-D
  end
  D-->E
  classDef red fill:#fdc
  class A red
```

What do you think?
