fix(MdSelect): Reactive options (#1447)

Make options reactive via a hidden node.

fix #1262

Author: VdustR

src/components/MdField/MdSelect/MdSelect.vue

@@ -33,10 +33,14 @@
         :md-content-class="mdClass"
         :style="menuStyles"
         @enter="onMenuEnter">
-        <slot />
+        <slot v-if="showSelect" />
       </md-menu-content>
     </keep-alive>
 
+    <div v-if="!showSelect" v-show="false">
+      <slot />
+    </div>
+
     <input class="md-input-fake" v-model="model" :disabled="disabled" readonly tabindex="-1" />
     <select readonly v-model="model" v-bind="attributes" tabindex="-1"></select>
   </md-menu>