solution to s9
solution for s10
solution for s11 using duplexer
solution for s12 Duplexer Redux

This module takes in a readable stream and passes that directly to the
duplexer as the readable entity.

The writable takes an input stream and passes it to through(write, end)

On write, through find the "country" form from the data and checks if
that entry already exists.  If it does, the count is incremented for
that entry.  If not, that entry is created with a count of 1.

On end, through calls the "counter" stream's setCount() and passes in
the counts object that the method has been creating.