Merge pull request #171 from furey/master

API note RE: extracting the source plain data object via JSON parse/stringify

Author: yyx990803

src/api/index.md

@@ -371,6 +371,8 @@ type: api
 
   Properties that start with `_` or `$` will **not** be proxied on the Vue instance because they may conflict with Vue's internal properties and API methods. You will have to access them as `vm.$data._property`.
 
+  If required, a deep clone of the original object can be obtained by passing `vm.$data` through `JSON.parse(JSON.stringify(...))`.
+
 - **Example:**
 
   ``` js