code: function task(callback) {
  	// spawn phantomas process
  	var child = phantomas(url, options, function(err, data, results) {
  		callback(
  			null, // pass null even in case of an error to continue async.series processing (issue #380)
  			[err, results]
  		);
  	});
  
  	child.on('progress', function(progress, inc) {
  		if (bar) {
  			bar.tick(inc);
  		}
  	});
  
  	// pipe --verbose messages to stderr
  	child.stderr.pipe(process.stderr);
  }
docs: perform a single run