psql: edit function, show function commands patch

Hello

this simple patch allow to specify cursor row when some function is
opened in editor.

\e aaa.txt
\a aaa.txt 3 ... move cursor on 3nd line of text
\ef foo
\ef foo 3 ... move cursor on 3nd line of function body
\sf foo ... show function body
\sf+ foo ... show function body - use line numbers
\sf[+] foo n ... show function body from line n

postgres=# \sf foo

CREATE OR REPLACE FUNCTION public.foo()
RETURNS integer
LANGUAGE plpgsql
AS $function$
begin
return 10/0;
end;
$function$

postgres=# \sf+ foo

**** CREATE OR REPLACE FUNCTION public.foo()
**** RETURNS integer
**** LANGUAGE plpgsql
1 AS $function$
2 begin
3 return 10/0;
4 end;
**** $function$

Regards
Pavel Stehule