transitions mixin and global variable, add it to buttons

dist/css/bootstrap.css

@@ -2112,6 +2112,9 @@ select[multiple].form-group-lg .form-control {
   background-image: none;
   border: 1px solid transparent;
   border-radius: .2rem;
+  -webkit-transition: all .2s ease-in-out;
+       -o-transition: all .2s ease-in-out;
+          transition: all .2s ease-in-out;
 }
 .btn:focus,
 .btn:active:focus,

docs/dist/css/bootstrap.css

@@ -2112,6 +2112,9 @@ select[multiple].form-group-lg .form-control {
   background-image: none;
   border: 1px solid transparent;
   border-radius: .2rem;
+  -webkit-transition: all .2s ease-in-out;
+       -o-transition: all .2s ease-in-out;
+          transition: all .2s ease-in-out;
 }
 .btn:focus,
 .btn:active:focus,

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