transitions mixin and global variable, add it to buttons

less/buttons.less

@@ -19,6 +19,7 @@
   white-space: nowrap;
   .button-size(@padding-base-vertical; @padding-base-horizontal; @font-size-base; @line-height-base; @border-radius-base);
   user-select: none;
+  .transition(all .2s ease-in-out);
 
   &,
   &:active,

less/mixins.less

@@ -9,6 +9,10 @@
   box-shadow: @arguments;
 }
 
+.transition(@transition) when (@enable-transitions = true) {
+  transition: @arguments;
+}
+
 .render-gradient(@start; @end) when (@enable-gradients = true) {
   #gradient > .vertical(@start-color: @start; @end-color: @end);
   background-color: @end;

less/variables.less

@@ -43,6 +43,7 @@
 @enable-rounded:            true;
 @enable-shadows:            true;
 @enable-gradients:          true;
+@enable-transitions:        true;
 
 
 //== Typography