How to change the resolution of the png image?

Hey, thank you very much for maintaining this package: 

I am using the code like this: 
```
from main import main
from pycallgraph2 import PyCallGraph
from pycallgraph2.output import GraphvizOutput

with PyCallGraph(output=GraphvizOutput()):
    main()
```

How do I change the resolution of ```pycallgraph.png```? I don't find the option in the ```GraphvizOutput``` class or the ```Config``` class.