date: support setting incoming timezone within --date

[jfinkels]

Steps to reproduce:

TZ=America/New_York date -d 'TZ="Europe/Paris" 2017-01-01 00:00:00'

This is asking the question "When it is midnight on January 1, 2017 in Pairs, what is the date/time in New York?"

What happens now: uutils date fails to parse the string:

date: invalid date 'TZ="Europe/Paris" 2017-01-01 00:00:00'

What I expected to happen: GNU date prints the correct date and time:

Sat Dec 31 06:00:00 PM EST 2016

Notes: this is causing a failure in the GNU test file tests/date/date-tz.sh. This behavior is documented at https://www.gnu.org/software/coreutils/manual/html_node/Specifying-time-zone-rules.html