Make it able to track progress

[skogsmaskin]

Hi rse!

I've made some changes to the code in order to facilitate progress tracking and better logging as the job progresses.

I've changed the .execute code to perform spawn instead of execFile, which allows us to handle stdout and stderr as they arrive. These can now be caputred line by line by chaning the Prince call with:

  .on('stderr', function(line, prince) { ... })
  .on('stdout', function(line, prince) { ... })

Registering multiple event handlers on the same events is possible too.

Price supports the option --structured-log=progress which can be used to track progress of the job.
This can now be achieved like this:

Prince()
  .inputs(...)
  .output(...)
  .option('structured-log', 'progress')
  .on('stderr', trackProgress)
  .execute()

The trackProgress function may look something like this:

function trackProgress(line, prince) {
  if (line.substring(0, 4) === 'prg|') {
    const value = parseInt(line.split('|')[1], 0)
    console.log(`${value}% done`)

    // This is also possible (for example value only)
    if (value > 49) {
      prince.off('stderr', this);
      console.log('I don't want to track it no more than 50%!')
    }

  }
}

Not sure if this is an ideal implementation though. First of, Prince doesn't seem to output anything to stdout, but I implemented a stdout-handler as well, as I'm not sure if other versions of prince may output something to stdout.

Also, it is maybe a little strange with .on handlers when things like error is handled as a promise rejection (kind of a mix of two different patterns). In this regard the chaining function name could be called .onOutput instead. However I find the .on pattern to be familiar and flexible for possible future expansions.

What do you think?