Improvement idea: ability to skip some items in the output tuple/list

When an item in the data stream is used as a count for a later item, it's often not needed in the output of `unpack`.
```
_, items, _, items2= unpack(data, "H /p1[H] H /p1[BH]")
```

It would be great to signal that in the format to avoid placeholders in the assignment

Mock example:
```
items, items2= unpack(data, "H* /p1[H] H* /p1[BH]")
```

