Add css-timing-1.vim

See also: http://www.w3.org/TR/2017/WD-css-timing-1-20170221/

test/test.html

@@ -741,6 +741,11 @@
 }
 </style>
 <style>
+.timing-1 {
+  display: frames(2);
+}
+</style>
+<style>
 .transforms-1 {
   backface-visibility: flat;
   perspective: preserve-3d;