date: add support for %q (quarter) in format string

Steps to reproduce:
```
date --date '2016-6-1' +%q
```

What happens now: uutils `date` terminates with an error message:
```
date: invalid format %q
```

What I expected to happen: GNU `date` prints the quarter number:
```
2
```

Notes: this is causing a failure in the GNU test file `tests/date/date.pl`.