Feature request: get original user agent.

The browser user agent has a lot of information. If it was overriden by OverrideUserAgent cordova preference then there is no way to get the original user agent from javascript. It would be nice if this plugin would also have a property for the original user agent.

### ios possible implementation
https://github.com/apache/cordova-ios/blob/master/CordovaLib/Classes/Public/CDVUserAgentUtil.m#L37

### android possible implementation
```
String userAgent = System.getProperty( "http.agent" );
```
https://stackoverflow.com/questions/4103963/is-there-a-way-to-obtain-the-default-user-agent-string-aside-from-webview-getset/10248817#10248817