`ent list` should take a git-like --pretty=format:

The immediate use is:
* list all issues in state=inprogress, but only tell me the assignees of each

I currently do this with a slow and awkward shell script:
```
for ISSUE_ID in $(ent list state=inprogress | grep '  ' | cut -f 1 -d ' '); do
    ent show "${ISSUE_ID}" | grep -i assign
done
```