print return value of main-tasks

This is a very minor change but it makes it possible to use the result of a task both when calling it from another task

```python
@task
def get_version():
    return 123

@task
def foo():
    version = get_version()
```

and from the shell

```bash
$> inv get_version
123
```

If there are no objections to this, I could add some tests for it.