Should comtypes support default properties

Default properties add syntactic sugar to interfaces, as described [here](http://msdn.microsoft.com/en-us/library/az06zx4y%28v=vs.90%29.aspx). `Item` is usually a default property.

Comtypes always treats `Item` as a default property, as defined [here](https://github.com/enthought/comtypes/blob/b8179ee7e6036bc00d8493d5be5d09a0ca292c9b/comtypes/__init__.py#L369), but perhaps should support default properties in a more general way.