fix: form ant-ref directive bug(#20)

Author: tangjinzhou

components/form/index.jsx

@@ -1,4 +1,8 @@
+import Vue from 'vue'
 import Form from './Form'
+import antRefDirective from '../_util/antRefDirective'
+
+Vue.use(antRefDirective)
 
 export { FormProps, FormComponentProps, FormCreateOption, ValidateCallback, ValidationRule } from './Form'
 export { FormItemProps } from './FormItem'