PATCH: psql boolean display

Hello all,

I am providing a patch to allow you to change the output of a boolean
value in psql much like you can do with NULL. A client requested this
feature and we thought it may appeal to someone else in the community.

The patch includes updated docs and a regression test. The code
changes themselves are pretty simple and straightforward.

Example from the regression test:

SELECT true, false;
bool | bool
------+------
t | f
(1 row)

\pset booltrue 'foo'
\pset boolfalse 'bar'
SELECT true, false;
bool | bool
------+------
foo | bar
(1 row)

\pset booltrue 't'
\pset boolfalse 'f'
SELECT true, false;
bool | bool
------+------
t | f
(1 row)

As always, comments welcome.

Thanks.