Add 'numeric' value for 'system' descriptor

With Vim 7.4.640, the keyword seems not to be treated by neither Vim's
css.vim nor this extension.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>

test/test.css

@@ -121,6 +121,7 @@
   display: symbols("*" "\2020" "\2021" "\A7");
   display: bullets;
   display: words;
+  display: numeric;
 }
 
 .display {

test/test.html

@@ -131,6 +131,7 @@
         display: symbols("*" "\2020" "\2021" "\A7");
         display: bullets;
         display: words;
+        display: numeric;
       }
 
       .display {