Add allow clear class

Author: yesmeck

assets/select.less

@@ -13,6 +13,12 @@
   vertical-align: middle;
   color: #666;
 
+  &-allow-clear {
+    .@{selectPrefixCls}-selection--single .@{selectPrefixCls}-selection__rendered {
+      padding-right: 40px;
+    }
+  }
+
   ul, li {
     margin: 0;
     padding: 0;

src/Select.jsx

@@ -880,6 +880,7 @@ class Select extends Component {
       // [`${prefixCls}-combobox`]: isCombobox(props),
       [`${prefixCls}-disabled`]: disabled,
       [`${prefixCls}-enabled`]: !disabled,
+      [`${prefixCls}-allow-clear`]: !!props.allowClear,
     };
 
     const clear = (<span