pgsql: Add a CONTINUE statement to PL/PgSQL, which can be used to begin

Log Message:
-----------
Add a CONTINUE statement to PL/PgSQL, which can be used to begin the
next iteration of a loop. Update documentation and add regression tests.
Patch from Pavel Stehule, reviewed by Neil Conway.

Modified Files:
--------------
pgsql/doc/src/sgml:
plpgsql.sgml (r1.73 -> r1.74)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.73&r2=1.74)
pgsql/src/pl/plpgsql/src:
gram.y (r1.76 -> r1.77)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.76&r2=1.77)
pl_exec.c (r1.146 -> r1.147)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.146&r2=1.147)
pl_funcs.c (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c.diff?r1=1.44&r2=1.45)
plpgsql.h (r1.63 -> r1.64)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.63&r2=1.64)
scan.l (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l.diff?r1=1.40&r2=1.41)
pgsql/src/test/regress/expected:
plpgsql.out (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.34&r2=1.35)
pgsql/src/test/regress/sql:
plpgsql.sql (r1.29 -> r1.30)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.29&r2=1.30)