Allow registering a hook to modify/pre-process String values read

(note: counterpart to https://github.com/FasterXML/jackson-core/issues/355 -- see more discussion there)

For some use cases -- for example, variable substitution -- it would be beneficial to be able to register a handler that can replace parts of incoming String values before other databinding functionality handles it.
Since there are various ways this could occur, I'll add a databinding issue here; the original `jackson-core` one (see above) may be most likely way to get there, but even if so there probably needs to be a way to register such handler on per-call basis for `ObjectReader`s (and/or maybe `ObjectMapper`).