Allow reading "at" location (with `JsonPointer`)

[cowtowncoder]

It would be nice to allow reading a subset of document, similar to full Jackson's

Value v = mapper.readerFor(Value.class).at("/some/path").readValue(source);

[cowtowncoder]

While nice, one problem is jackson-jr API itself: it is missing necessary counterpart to ObjectReader, since adding overloads for readXxx() would be impractical due to combinatorial explosion.
So might be more for 3.x