Add a subscriber constructor.

Author: walterpinson

src/Core/Domain/Models/Subscriber.cs

@@ -6,5 +6,13 @@ public class Subscriber: ISubscriber
     {
         public Guid Id { get; set; }
         public string EmailAddress { get; set; }
+
+        public Subscriber(string emailAddress)
+        {
+            if (string.IsNullOrEmpty(emailAddress)) throw new ArgumentException("message", nameof(emailAddress));
+
+            EmailAddress = emailAddress;
+            Id = Guid.NewGuid();
+        }
     }
 }