Add encoding support to COPY

Today on IRC, someone was wondering what the preferred method of
exporting data in a specific encoding via COPY was. They reply was
wrapping the COPY command in "set client_encoding='foo';", which made
me wonder how hard it would be to add an additional WITH parameter to
the actual COPY statement to specify the encoding, a la:
[ [ WITH ]
[ BINARY ]
[ OIDS ]
[ DELIMITER [ AS ] 'delimiter' ]
[ ENCODING [ AS ] 'charset' ]
[ NULL [ AS ] 'null string' ]
[ CSV [ HEADER ]
[ QUOTE [ AS ] 'quote' ]
[ ESCAPE [ AS ] 'escape' ]
[ FORCE QUOTE column [, ...] ]

Any objections? It seems like a cleaner solution client side than
issuing multiple calls to set the client_encoding. If there are no
objections, I can attempt to prepare a patch for the next commitfest.

David Blewett