feat(MdSnackbar): keep alive if its persistent (vuematerial#1561)

* fix(MdSnackbar): keep alive if its persistent

* docs(MdSnackbar): add `md-persistent` prop

Author: Samuell1

docs/app/pages/Components/Snackbar/Snackbar.vue

@@ -45,6 +45,12 @@
             description: 'Sets the duration in milliseconds before close the snackbar.',
             defaults: '4000'
           },
+          {
+            name: 'md-persistent',
+            type: 'Boolean',
+            description: 'This will make the snackbar persistent in your application, even changing routes.',
+            defaults: 'false'
+          },
           {
             name: 'md-position',
             type: 'String',

src/components/MdSnackbar/MdSnackbar.vue

@@ -50,7 +50,7 @@
     watch: {
       mdActive (isActive) {
         if (isActive) {
-          createSnackbar(this.mdDuration, this).then(() => {
+          createSnackbar(this.mdDuration, this.mdPersistent, this).then(() => {
             this.$emit('update:mdActive', false)
             this.$emit('md-opened')
           })

src/components/MdSnackbar/MdSnackbarQueue.js

@@ -1,7 +1,7 @@
 let currentSnackbar = null
 let timeout = null
 
-function createPromise (duration, context) {
+function createPromise (duration, persistent, context) {
   return new Promise(resolve => {
     currentSnackbar = {
       destroy: () => {
@@ -13,7 +13,9 @@ function createPromise (duration, context) {
     if (duration !== Infinity) {
       timeout = window.setTimeout(() => {
         destroySnackbar()
-        context._vnode.componentInstance.initDestroy(true)
+        if (!persistent) {
+          context._vnode.componentInstance.initDestroy(true)
+        }
       }, duration)
     }
   })