Add css3-positioning.vim

See also: http://www.w3.org/TR/2015/WD-css3-positioning-20150203/

test/test.html

@@ -336,6 +336,13 @@
         display: verso;
       }
 
+      .positioning {
+        offset-before: sticky;
+        offset-end: auto;
+        offset-after: auto;
+        offset-start: auto;
+      }
+
       .preslev {
         presentation-level: increment;
       }