Add passthrough action for importance attribute

This will allow us to mark deprecated parameters and extension points in HTML output per
#648 (comment)

Signed-off-by: Roger Sheen <[email protected]>

Author: infotexture

resources/html.ditaval

@@ -3,4 +3,5 @@
 <val>
   <prop action="include" att="deliveryTarget" val="html"/>
   <prop action="exclude" att="deliveryTarget" val="pdf"/>
+  <prop action="passthrough" att="importance"/>
 </val>

resources/site.ditaval

@@ -3,5 +3,6 @@
 <val>
   <prop action="include" att="deliveryTarget" val="html"/>
   <prop action="exclude" att="deliveryTarget" val="pdf"/>
+  <prop action="passthrough" att="importance"/>
   <prop action="passthrough" att="platform"/>
 </val>