Expose image region extraction code as CLI

For implementations using the bashlib API, it would be useful to have a command to extract a specific block/line from a PAGE, e.g.

```
ocrd workspace extract --element-id="line123" --page-id="page321" --output=line123.tiff
```

c.f. https://github.com/OCR-D/ocrd_olena/pull/2#discussion_r304248793
