Allow filtering content written via `JsonGenerator` by specifying `JsonPointer`

[cowtowncoder]

(similar to #137, different direction)

For many dynamic use cases it would be nice to be able to only include a subset (subtree) of content that is being written, so that, for example, a JsonPointer defines portion of a POJO is included in the output actually generated. While it would be more efficient to try to select subset before generation ability to define subsets dynamically make it more convenient to separate out parts.

[cowtowncoder]

Implemented, by adding new package com.fasterxml.jackson.core.filter and classes:

• TokenFilter for specifying filtering criteria; JsonPointerBasedFilter as a sample implementation
• FilteringGeneratorDelegate for actual generator that uses specified filtering strategy (TokenFilter)

[tlrx]

Awesome! :)