Allow ignoring multiple target properties

It would be great if mapstruct provided a way of ignoring multiple target properties.

Instead of writing this
```
@Mapping(target = "property1", ignore = true)
@Mapping(target = "property2", ignore = true)
@Mapping(target = "property3", ignore = true)
```

I would prefer
```
@Mapping(target = {"property1", "property2", "property3"}, ignore = true)
```