Endless Game. Allow user to continue the game.

- main.css isn't compiled
- a special class `.tile-gold` is added to tiles above 2048

style/main.scss

@@ -218,6 +218,10 @@ hr {
         @include button;
         margin-left: 9px;
         // margin-top: 59px;
+
+	&.keep-playing-button {
+	  display: none;
+	}
       }
 
       @include animation(fade-in 800ms ease $transition-speed * 12);
@@ -226,6 +230,10 @@ hr {
       &.game-won {
         background: rgba($tile-gold-color, .5);
         color: $bright-text-color;
+
+	a.keep-playing-button {
+	  display: inline-block;
+	}
       }
 
       &.game-won, &.game-over {