Created Feature: Add Ignore methods to lightweight conventions [732]

Ignore methods should be added to make it easier to create lightweight conventions that exclude things from the model. For example:

modelBuilder.Entities().Where(t => t.Namespace == "Some.External.Namespace").Configure(e => e.Ignore());
modelBuilder.Properties().Where(p => p.GetCustomAttributes<NonSerializedAttribute>().Any()).Configure(p => p.Ignore());