add a command to generate proxy template

`pub run new_proxies a.b.MyClass MyClass2` will generate in the console output :

```dart
abstract class MyClass extends JsInterface {
  factory MyClass() => new MyClassImpl();
  MyClass.created(JsObject o) : super.created(o);
}

@JsProxy(constructor: 'a.b.MyClass')
class MyClassImpl extends MyClass {
  factory MyClassImpl() => new JsInterface(MyClassImpl, []);
  MyClassImpl.created(JsObject o) : super.created(o);
  noSuchMethod(i) => super.noSuchMethod(i);
}

abstract class MyClass2 extends JsInterface {
  factory MyClass2() => new MyClass2Impl();
  MyClass2.created(JsObject o) : super.created(o);
}

@JsProxy(constructor: 'MyClass2')
class MyClass2Impl extends MyClass2 {
  factory MyClass2Impl() => new JsInterface(MyClass2Impl, []);
  MyClass2Impl.created(JsObject o) : super.created(o);
  noSuchMethod(i) => super.noSuchMethod(i);
}
```
I'd be nice to eventually create the Dart files in the right directories based on the qualified name.