Support AugAssign

I wanted this to be a part of #40, But I am wrapping that up and this will be added later.

Currently, we do not support AugAssign operations. Once done, we can save some verbosity.

```py
x = x + 1

# will become

x += 1
```

This will, however, come up when we'll try to port various utilities to PythonBPF, and I am sure that this will be added in sooner or later.