Add @JsonIgnoreProperties.Value to support merging of settings

[cowtowncoder]

(note: related to #82, pre-requisite)

In order to allow merging of settings from multiple sources and/or levels (and for Jackson 2.8, specifically for multiple sources with the new "property config overrides"), it is necessary to introduce separate value object, similar to how JsonInclude.Value and JsonFormat.Value work.
So let's add this in 2.8; future work is likely needed next for @JsonPropertyOrder but one step at a time.