add `bw items` to preview items

```
usage: bw items [-h] [-w DIRECTORY] [--repr] NODE

positional arguments:
  NODE                  list items for this node

optional arguments:
  -h, --help            show this help message and exit
  -w DIRECTORY, --write-file-previews DIRECTORY
                        create DIRECTORY and fill it with rendered file
                        previews
  --repr                show more verbose representation of each item
```