[Load Digraph/Design] preserve/remove subgraph keyword?

Possibly preserve subgraph keyword. If not present, should not add to file.

For example:

Input:
`b -- { c e };`

The dot read should not have "subgraph"
like:
```
b -- subgraph
{
c;
e;
}
```

Note:
**All the graphs generated from csdot should have that keyword.**