Support `objectMode` for one side only

[ip]

Should respect readableObjectMode and writableObjectMode options:

https://nodejs.org/dist/latest-v8.x/docs/api/stream.html#stream_object_mode_duplex_streams

[IronSavior]

I think this is already supported by virtue of this line.

[freddi301]

const duplex = duplexify(encoder, decoder, {
  objectMode: true,
  writableObjectMode: true,
  readableObjectMode: true,
})
duplex.writableObjectMode // is undefined
duplex.readableObjectMode // is undefined