Listener for unmanaged file extensions.

Hi, I think it could be very useful to manage other types of file.

I tried to open txt files, mp3 files and it seems fine. What about pdf, doc etc???

I would like to download (or open in a different app) for example pdf files but at the moment the listener cannot listen to this extensions...
```
flutterWebviewPlugin.onUrlChanged.listen((String url) async {
if (url.contains('.pdf')) {
print(url); //This print is never done
}
if (url.contains('.doc')) {
print(url); //This print is never done
}
if (url.contains('mailto:')) {
print(url); //This print is ok
}
if (url.contains('.txt')) {
print(url); //This print is ok
}
if (url.contains('.mp3')) {
print(url); //This print is ok
}
}
```