Added the very useful .animation-fill-mode() mixin

I think that's the only animation property that was missing and it's quite useful.

https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode

less/mixins.less

@@ -243,6 +243,10 @@
   -webkit-animation-direction: @direction;
           animation-direction: @direction;
 }
+.animation-fill-mode(@fill-mode) {
+  -webkit-animation-fill-mode: @fill-mode;
+          animation-fill-mode: @fill-mode;
+}
 
 // Backface visibility
 // Prevent browsers from flickering when using CSS 3D transforms.