process object to get lastOutputTime

[lichnow]

Description
I run watch webpack build by symfony process,but when i can not check if is building before wait.so ,i should a public method in process class to get lastOutputTime to check it.

Example

$this->runner($params, $messages, true,function(Process $process,$progressBar){
                $process->setTimeout(null);
                $process->start();
                $process->wait(function ($type, $buffer) use($progressBar) {
                    if (microtime(true) - $process->getLastOutputTime() > 10){
                        $this->output->writeln("<fg=yellow>start to watch</>");
                    }
                });
            });

[fabpot]

@lichnow Would you like to submit a pull request?