Added a validate target to check that the SGML markup is valid without
the overhead of running Jade.

Added an ${EXTRA_CATALOGS} variable if your document needs to be
processed with additional SGML catalogs (currently not used by anything,
but might be some day).

Switched the meaning of ${JADEFLAGS} and ${JADEOPTS} for consistency. All
user-tweakable program options now match ${*FLAGS}.