New step: `progress`

[wojtekmach]

Requires #10.

When running in IEx, by default we'll simply write to stdout:

iex> Enum.each(1..99, fn i -> IO.write "\r" <> String.pad_leading("#{i}%", 4, " ") ; Process.sleep(50) end)
  1%.. 2%..

there will also be an option to pass a callback to e.g. send a message somewhere.