Invert a Hardwired Dependency

PriceNotifier currently news up SmtpEmailer directly (provided). Introduce IEmailer (Send(string, string)), make PriceNotifier take IEmailer via constructor, and implement a LoggingEmailer that records "log:<to>:<body>" into its Log list. ``csharp public interface IEmailer { void Send(string to, string body); } public sealed class SmtpEmailer : IEmailer { public void Send(...) { /* real */ } } public sealed class PriceNotifier { public void Notify(string email, string product, decimal price); } ``