Style: callback to have explicit returns

Currently the behavior with callbacks is inconsistent with explicit return for callbacks winning slightly:

```
ag next | grep -v function | grep -v return | wc # => 159
ag next | grep -v function | grep return | wc # => 191
```

Argument for explicit return on callback is that you do not want any code executed after a `callback()` which sets it apart from any other function calls. Making this explicit clarifies the flow.