title: Using nested property string with automapper MapFrom
body: <p>I'd like to use a string that represents some navigation properties on my class for the mapping. I've also tried converting the string to a PropertyExpression but have not been able to wire it up correctly.
  Is it possible to use automapper in the way demonstrated below?</p>
  
  <pre><code>var config = new MapperConfiguration(cfg =&gt;
  {
      cfg.CreateMap&lt;SourceClass, DestClass&gt;()
         .ForMember(targetField, "prop1.nested1.nested2";
  });
  </code></pre>