Add support for registering default builders

[basverweij]

Default builders are checked for applicability when a builder is missing for a non-assignable mapping.

Add WithDefaultBuilder, and WithDefaultBuilderFrom(x) methods to IMappingBuilder interface. Re-use DelegateBuilders from MappingRuleBuilderFrom(x)To(y) classes.

Create AddDefaultBuilderValidator validation rule, by default performed before the AddAutoMapper rule.