title: exclude specific fields in mapstruct
body: <p>While I am trying to create a <code>mapper</code> between two <code>classes</code> in <a href="http://mapstruct.org/" rel="noreferrer"><code>mapstruct</code></a>,<br>
  I am getting a <code>warning</code> when I compile my code : </p>
  
  <pre><code>src/main/java/mapstruct/DogMapper.java:15: warning: Unmapped target property: "otherField".
      Cat convert(Dog dog);
          ^
  1 warning
  </code></pre>
  
  <p>This are the two objects I am trying to map between :  </p>
  
  <p><strong>Dog</strong></p>
  
  <pre><code>@Getter
  @Setter
  public class Dog {
      private String say;
  }
  </code></pre>
  
  <p><strong>Cat</strong></p>
  
  <pre><code>@Getter
  @Setter
  public class Cat {
      private String say;
      private String otherField;
  }
  </code></pre>
  
  <p>And this is my <strong>Mapper</strong></p>
  
  <pre><code>@Mapper
  public interface DogMapper {
      DogMapper mapper = Mappers.getMapper( DogMapper.class );
  
      @Mapping(source = "say", target = "say")
      Cat convert(Dog dog);
  }
  </code></pre>
  
  <p>I read the <a href="http://mapstruct.org/documentation/stable/reference/html/" rel="noreferrer"><code>mapstruct docs</code></a>, and I know i can exclude this specific field in many ways : </p>
  
  <pre><code>@Mapping(ignore = true, target = "otherField")
  </code></pre>
  
  <p>Or by this way : </p>
  
  <pre><code>@Mapper(unmappedTargetPolicy = ReportingPolicy.IGNORE)
  </code></pre>
  
  <p>But my purpose in the end is to exclude the specific field called <code>otherField</code>,<br>
  from all my mappers, but not to exclude other field that I am not using.</p>
  
  <p>Is there any way to achieve that?</p>