增加placeholder

src/form/textarea.js

@@ -14,6 +14,7 @@ class TextArea extends Component {
       attribute: false
     },
     type: '',
+    placeholder: '',
     autofocus: false,
     readOnly: false,
     disabled: false,
@@ -217,6 +218,7 @@ class TextArea extends Component {
           :value=${this.value}
           ref="textarea"
           spellcheck="false"
+          placeholder=${this.placeholder}
           readonly=${this.readOnly}
           disabled=${this.disabled}
           maxlength=${this.maxlength}