Compute coreos version for a date or the date of a coreos version.
Arguments are passed to `date` except for `-v` which takes an optional
version value and converts it to a date. `-v` must be the first arg.
$ ./core_date
945
$ ./core_date -d wed
947
$ ./core_date -v
Fri Jan 29 00:00:00 UTC 2016
$ ./core_date -v +%D
01/29/16
$ ./core_date -v 1000
Sun Mar 27 00:00:00 UTC 2016
$ ./core_date -v 1000 +%D
03/27/16
