provide mechanisms to forward output from jnlp app

[klarose]

A misbehaving application may log information to stderr, or even stdout.
Sometimes it is useful to collect this information for diagnostic
purposes. Provide a mechanism to capture this output in a customizable
fashion using callback functions invoked on a goroutine with the
stdin/stderr pipe returned from the exec.Cmd we launch.

An example of using this:

// makeLauncher makes a launcher configured to forward the launched applications stdout to logrus's Info level, and stderr to logrus's Error level.
func makeLauncher() launcher.Launcher {
	options := &launcher.Options{
		StdoutHandler: pipeLogger(logrus.Info),
		StderrHandler: pipeLogger(logrus.Error),
	}
        jnlpLauncher := jnlp.NewLauncher()
        jnlpLauncher.SetOptions(options)
       return jnlpLauncher
}

func pipeLogger(outFunc func(args ...interface{})) func(io.ReadCloser) {
	return func(input io.ReadCloser) {
		defer input.Close()
		scanner := bufio.NewScanner(input)
		for scanner.Scan() {
			outFunc(scanner.Text())
		}
	}
}

[klarose]

An alternative to this would be to add it to the interface. I chose the options to limit the requirements of the interface, but I'm good with either approach.