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
}
}
```

Start from the flutterWebviewPlugin.onUrlChanged listener shown in the issue and reproduce navigation to PDF and DOC URLs, comparing it with the working mailto, TXT, and MP3 cases. Done means the listener can observe unmanaged file-extension URLs so the application can download them or open them in another app.

Vom Indexierungsmodell aus dem Issue-Text verfasst.