\timing interval

\timing is great.
\timing is helpful.
\timing has made me really good at mentally estimating numbers modulo
3600000.

Wouldn't it be great if we had a way of printing timing in more human
friendly formats?

Attached is a patch that allows the following (new/interesting bits in
bold):

# \timing off
Timing is off.
# select pg_sleep(1);
pg_sleep
----------

(1 row)

# \timing
Timing is on.
# select pg_sleep(1);
pg_sleep
----------

(1 row)

Time: 1002.959 ms
*# \timing interval*
*Timing is interval.*
# select pg_sleep(1);
pg_sleep
----------

(1 row)

*Time: 00:00:01.003*
# \timing
Timing is off.

As demonstrated, "interval" toggles to "off". There is no way to toggle to
"interval".

I'm pretty flexible on how something like this gets invoked. We could leave
timing alone and create a format variable. We could actually leverage the
pre-existing interval-to-string code, etc.

Note: the current patch includes no doc changes. I'd figure I'd wait to do
that after this patch or another gains some traction.