Add relabel functinoality for Darknet (YOLO) annotation files

Add the ability to relabel Darknet (YOLO) annotation files. This is essentially a matter of replacing the label index, which is the first field in the annotation lines of the file. For example, we might go from this:
```
3 0.4995 0.2012 0.3994 0.0456
```
to this:
```
2 0.4995 0.2012 0.3994 0.0456
```