Add option to only set target properties if it is null or default value

[moxplod]

My use case is the enrichment of data in a class and I have sources that are more reliable than others. So if I use a less reliable source, then I do not want to overwrite an existing property if it already has a non-default value.

Something like this -

                    if (source.Twitter != null && target.Twitter != default)
                    {
                        target.Twitter = source.Twitter;
                    }

I am not sure if this exists; I tried finding it in the documentation and did not find it.

[latonz]

This is not yet supported and would be a nice addition. Relates to the current MapperAttribute.AllowNullPropertyAssignment and #296.
A simple API for this could be introducing a MapperAttribute.MemberMappingPrecondition with enum { None = 0, SourceAndTargetNotEqual = 1 << 0, SourceNotNull = 1 << 1, TargetNotNull = 1 << 2 }