A basic application exploring ideas around plugins and extensions.

Observations:

• Use the plugin package to slowly build up an extension model similar to Eclipse's extension model.
• Use the inject.dart package for dependency injection.
• Unfortunately, inject.dart strips generic types off, so something like this doesn't seem to work:

@module
class ExtensionPointsModule {
    @provide
    ExtensionPoint<Command> provideCommandExtensionPoint() => new ExtensionPoint<Command>();
}