add user-select mixin

lib/mixins.less

@@ -147,8 +147,7 @@
           transition: @transition;
 }
 
-// Transform for scale and rotate
-// translate, rotate, scale -- need to finalize
+// Transformations
 .rotate(@degrees: 5deg) {
   -webkit-transform: rotate(@degrees);
      -moz-transform: rotate(@degrees);
@@ -171,6 +170,15 @@
           transform: translate(@x, @y);
 }
 
+// User select
+// For selecting text on the page
+.user-select(@select) {
+  -webkit-user-select: @select;
+     -moz-user-select: @select;
+       -o-user-select: @select;
+          user-select: @select;
+}
+
 // Background clipping
 // Heads up: FF 3.6 and under need padding instead of padding-box
 .background-clip(@clip) {