Support saving a snapshot of the output to a file

[dsidirop]

Something like:

      nats-top    -o    snapshot.txt
      nats-top    -o    -    # this prints to standard output

This will save the first "terminal output" generated by nats into the file 'snapshot.txt' and then nats-top will exit.

[dsidirop]

@wallyqs New pull request up:

#58