feature request - builder with one field changed

in our code we end up writing
```
value = myClass.newBuilder(value).withFoo(new Foo)
```
quite a lot

it would be clearer to have additional methods generated so we could do

```
value = value.copyWithFoo(new Foo());
```

particually when you end up nesting the builders if I want to change a.b.c type 