[Document] add form components english document (#199)

* [Document] add english document of Checkbox

* [Document] add english document of Field

* [Document] add english document of NumberKeyboard

* [bugfix] NumberKeyboard should not dispaly title when title is empty

* [Document] add english document of PasswordInput

* [Document] add english document of Radio

* [document] add english document of Switch

* [bugfix] remove redundent styles in english document

* [Document] fix details

* fix Switch test cases

packages/number-keyboard/index.vue

@@ -10,7 +10,7 @@
       @touchcancel="blurKey"
       @animationend="onAnimationEnd"
     >
-      <div class="van-number-keyboard__title van-hairline--top">
+      <div class="van-number-keyboard__title van-hairline--top" v-if="title">
         <span>{{ title }}</span>
       </div>
       <i 
@@ -36,10 +36,7 @@ export default {
       type: String,
       default: ''
     },
-    title: {
-      type: String,
-      default: '安全输入键盘'
-    },
+    title: String,
     zIndex: {
       type: Number,
       default: 100