Support for WebDriver

[tokahuke]

The ideia of this PR is to make lopez to be able to connect to an external WebDriver compatible server to download and analyze pages. No, no Selenium in the mix, athough you can use Selenium WebDriver as the external server (or chromedriver, or geckodriver). Th reason: WebDriver is a W3C recommendation, so, let's stick to the standards.

The main objectives are:

• [ ] create an abstraction to allow multiple download methods in the same worker.
• [ ] implement the WebDriver downloader using the fantoccini crate.
• [ ] expose a CLI parameter to configure remote connections.
• [ ] extend LCD to allow hte user to configure when WebDrievr should be used instead of the default downloader.

[tokahuke]

Oh, crap! Committed a lot of stuff to that branch. Ignore PR #10 and create a new one!