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