Add new properties in `@JsonSetter`: `nulls`/`contentNulls`

[cowtowncoder]

New properties are needed by @JsonSetter, to support:

• [https://github.com/Use @JsonSetter(nulls=...) to specify handling of null values during deserialization jackson-databind#1402]: configurable handling of incoming null values

NOTE: earlier plans to add support here for:

• [https://github.com/Add support for @JsonMerge to allow "deep update"  jackson-databind#1399]: support for merging of property values

was CANCELLED in favor of #113.