Expose both readable and writable objectMode

[rgarcia]

Currently the only way to have a mixed transform that is objectMode=true on one side and objectMode=false on the other side is to make your own stream subclass and set _{readable,writable}State.objectMode in the constructor: http://nodejs.org/api/stream.html#stream_state_objects

This would allow us to properly support of transformations that map js objects to buffer/string values and vice versa.