Implement methods of an interface

[jfernandez]

When a contract implements an interface, like NonFungibleToken, the VS Code extension should give the user an option to create stubs for implemented methods. Currently the DevEx isn't great and requires you to copy/paste code from the interface. This is a very useful feature in IDE's like Intellij: https://www.jetbrains.com/help/idea/implementing-methods-of-an-interface.html