Consider supporting @JsonIgnore on field level

[philippd]

If a field is annotated with JsonIgnore (from Jackson), it usually does not make sense to pass its constraints to the client and it could therefore be excluded from the JSON structure.

This is however really specific to users of the Jackson JSON library which is not a requirement to use in conjunction with the valdr-bean-validation plugin. Still it could be a nice improvement of the manual exclusion of fields in an external configuration (see #18)

see also discussion in #11