feat(MdRipple): add a disabled class to make easy to distinguish

Author: marcosmoura

src/components/MdRipple/MdRipple.vue

@@ -1,5 +1,5 @@
 <template>
-  <div class="md-ripple" @touchstart.passive.stop="startRipple" @mousedown.passive.stop="startRipple">
+  <div class="md-ripple" :class="{ 'md-disabled': mdDisabled }" @touchstart.passive.stop="startRipple" @mousedown.passive.stop="startRipple">
     <transition name="md-ripple" appear @after-enter="clearWave" v-if="!mdDisabled">
       <span class="md-ripple-wave" :class="rippleClass" :style="waveStyles" v-if="animating" ref="rippleWave" />
     </transition>