API: rows.add() and row.add() return a rows() (or row()) extended object
with newly added rows

- Looking at fnAddData, and about to replace it, I realised that
  fnAddData returned the indexes of the newly added row while the new
  methods didn't. It makes sense for row.add() to return a row()
  extended object with the newly added rows so row().node() etc can be
  used. Also the indexes are in the inst, so that information is also
  available. Likewise is done for its plural counterpart.

- Did consider creating an `augment` static method for the API, and
  might yet do this, but this is the only place to use it at the moment.
  Possibly it might be useful for plug-ins, but before adding, lets see
  how the new API is used.