configurable chromedriver mirror URL

Provide a way to define a mirror url for the chromedriver download. This enables users in a internet-restricted environment (flaky internet connections or corporate proxies) to run a locally hosted mirror for chromedriver.

This implementation allows user to define an environment variable CHROMEDRIVER_CDNURL to override the default hostname used for downloading the chromedriver. This is similar to phantomjs' implementation (https://github.com/Medium/phantomjs/blob/master/install.js).