Add %CSS.ForbiddenProperties directive.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

library/HTMLPurifier/ConfigSchema/schema/CSS.ForbiddenProperties.txt

@@ -0,0 +1,13 @@
+CSS.ForbiddenProperties
+TYPE: lookup
+VERSION: 4.1.2
+DEFAULT: array()
+--DESCRIPTION--
+<p>
+    This is the logical inverse of %CSS.AllowedProperties, and it will
+    override that directive or any other directive.  If possible,
+    %CSS.AllowedProperties is recommended over this directive,
+    because it can sometimes be difficult to tell whether or not you've
+    forbidden all of the CSS properties you truly would like to disallow.
+</p>
+--# vim: et sw=4 sts=4