Add ability to use ipdb/pdb

Is it possible to add the ability to use ipdb/pdb. If I setup a tasks file like this:

``` python
from invoke import run, task


@task
def test():
    run("./manage.py test")

```

And then add this inside a test:

``` python
import ipdb; ipdb.set_trace()
```

I'm not able to run commands and I don't get a ipdb prompt. I was using Invoke 0.10.1 and Python 2.7.9.
