Support use of MapTo within custom mappings

[markashleybell]

So that we can use the default mapper to do most of the work, then just map the remaining bits, e.g:

Mapper.AddMapping<Person, PersonDTO>((source, destination) => {
    source.MapTo(destination);
    destination.Title = source.Name.Title;
    destination.Forename = source.Name.Forename;
    destination.Surname = source.Name.Surname;
});

At the moment this obviously causes problems due to recursive mapping lookups, so perhaps we can add an optional parameter to MapTo which tells it not to look for the custom mapping (and only use that optional param from within mapping closures)>