Add support in ObjectMapper for custom `ObjectReader`, `ObjectWriter` (sub-classes)

[cowtowncoder]

Currently ObjectReader and ObjectWriter are dataformat-agnostic, unlike ObjectMapper. That is, they are not designed for sub-classing. But since mapper itself is commonly sub-classed; mostly for convenience, but also to allow for dataformat-specific configurability, it'd make sense to allow some of that for readers and writers as well. Specific piece of functionality this would open is ability to re-configure data-format features on per-call basis.

But to get there, reader and writer objects need to add support for sub-classing; copy-constructor at least; and ObjectMapper also needs overridable methods to produce proper sub-classes as needed.