psql \G command -- send query and output using extended format

Hello!

Attached is a simple patch which adds a "\G" backslash command, that is
similar "\g" command. The difference is that "\G" prints results using
extended output format. After the query the format is reset to the original
value.

In other words command like:
SELECT * FROM pg_stat_activity\G
is similar to doing:
\x on
SELECT * FROM pg_stat_activity\g
\x on or off -- depending on the original state.

Rationale: switching format using "\x" for duration of single query I find
a bit uncomfortable. This is similar to "\g file" which is one-shot version
of "\o file" command.

Regards,
Dawid