psql: Add \gx command

psql: Add \gx command

It can often be useful to use expanded mode output (\x) for just a
single query.  Introduce a \gx which acts exactly like \g except that it
will force expanded output mode for that one \gx call.  This is simpler
than having to use \x as a toggle and also means that the user doesn't
have to worry about the current state of the expanded variable, or
resetting it later, to ensure a given query is always returned in
expanded mode.

Primairly Christoph's patch, though I did tweak the documentation and help
text a bit, and re-indented the tab completion section.

Author: Christoph Berg
Reviewed By: Daniel Verite
Discussion: https://postgr.es/m/20170127132737.6skslelaf4txs6iw%40msg.credativ.de