Add functions to load Chrome extensions.

I needed to be able to load Chrome extensions. I tried to follow the Firefox API. I also needed to make some changes to the `core` namespace.

    (def b (new-driver {:browser :firefox
                        :chrome-options (doto (chrome/new-chrome-options)
                                   (chrome/enable-extension "/path/to/extensions/chrome_extension_dir")

I'm not very confident with Clojure yet, so any feedback is appreciated :)