Add ICommand Example

_May require #1_

Would be cool to show how to expose an `ICommand` implementation which could be passed from the app (or a WRC, see #1) to the C# WRC project and used within a control of the Toolkit.

Start by reviewing issue #1 and the existing C++/WinRT UWP sample structure. Determine how an ICommand implementation can be passed from the app or a WRC into the C++/WinRT project and used by a control; done means the sample demonstrates that end-to-end.

由索引模型根据 Issue 内容生成。