Allow DataObject properties to be ignored by generated converter

[rgmz]

In certain circumstances it is desirable or necessary to omit a property from the generated converter.

For example, the following Kotlin DataObject will not compile successfully:

@DataObject(generateConverter = true)
class EnumSetDataObject() {
    var enumSet: Set<MyEnum> = emptySet()
    ...
}

error: Could not generate element for io.vertx.example.EnumSetDataObject: 
Illegal type ? extends io.vertx.example.MyEnum of kind WILDCARD

The desired effect (omitting enumSet from the converter) can be accomplished in Kotlin by using the @JvmField annotation, however, that is a language-specific solution.

I think that it would be useful to provide this functionality, either via the @GenIgnore annotation or a new @ConverterIgnore annotation.

[rgmz]

Update: one way of accomplishing this, in Kotlin, is to use the @JvmField annotation as described in #330.