Make it possible to supply ChromeOptions to start_chrome

Added an argument options to start_chrome(...). You can use it to supply the ChromeOptions when starting the browser. For example:

from selenium.webdriver import ChromeOptions
options = ChromeOptions()
options.add_argument('--start-maximized')
options.add_argument('--proxy-server=1.2.3.4:5678')
start_chrome(options=options)