add chromeFlags to Chromeless constructor

[royalgarter]

Example:

const chromeless = new Chromeless({ 
	debug: true, 
	port: PORT,
	chromeFlags: [
		'--window-size=1366,786',
		'--proxy-server=myproxy.com:3128',
		'--disable-gpu',
		'--headless',
	]
});

[cseeger]

Possible dupe of #192 ?