date: implement `-r`/`--reference`

From the GNU `date` [docs](https://www.gnu.org/software/coreutils/manual/html_node/Options-for-date.html#index-_002dr-15):
```
-r file
--reference=file

Display the date and time of the last modification of file, instead of the current date and time.
```