[PROPOSE] Add JSON API to delete complex graph with its associated graphs

This pull request is to make it possible to delete all the graphs on API like:
``` sh
curl -X POST -H "Content-Type: application/json" -d '{"delete_graph": "1"}' http://localhost:5125/json/delete/complex/a/b/c
```
although the default behavior will not be changed.

This feature is useful for those who often create and delete complex graphs.