Support type visitability (for JSON Schema module) for Scala Collections, Maps, Option

[cowtowncoder]

(note: copied from FasterXML/jackson-module-jsonSchema#71)

Looks like Scala types may not (fully?) support visitability, to allow things like JSON Schema generation.
Adding support is relatively easy, as it only requires implementation of method acceptJsonFormatVisitor(...) from JsonSerializer.

[christophercurrie]

Looks like it might be specific to collections types. This should be doable without much trouble, I'll look at it for 2.6.2.