Add mechanism, annotation, to handle "JSON-in-JSON" case

One common (anti?-)pattern that exists is for a JSON-String valued property to contain JSON-encoded value; effectively using double-encoding. It would be nice to be able to support such case automatically with just an annotation, like `@JsonDoubleEncoded` or something.
What this would do would be allow usage like:

``` java
public class Funny {
public int id;
@JsonDoubleEncoded
public Map attrs;
```

but produce something where `attrs` is actually JSON String, and not JSON Object; encoded further as JSON.

For extra points this should also work for other data-formats; but most likely would have to be limited to "in-kind" encoding as there isn't a good way to access other backends at the point where this is needed. For trans-coding (like JSON in XML, XML in JSON, JSON in Protobuf, whatever) custom handler would still be needed.
Note: although technically, at this point, producing JSON can actually be done from all other data-formats (due to original being JSON-centric), I don't think we should rely on this since going forward this invariant might not hold, and we want to lessen, not strengthen this dependency.