Allow filtering of content read via JsonParser by specifying JsonPointer to match

[cowtowncoder]

(note: whether filter should live here, or at jackson-databind is open to discussion)

Now that core module contains basic JsonPointer implementation (currently used by locating nodes in tree model), it would also make sense to allow basic filtering to be done using it. A natural (and very efficient) place to do that would be at streaming reader level.
So if possible, it would be good to add a delegate-style reader that takes a JsonPointer, and exposes matching subtree.

For bonus points one might be able to support set of filters, but that may or may not be practical; so start with a single pointer expression.

[cowtowncoder]

Implemented for 2.6.0.