Allow Preference for Route Connectivity

Just out of a minor concern for privacy, allowing users to specify which 
routes they'd like to hit to determine network connectivity. This is also
since google.com can be available in a capacitively-blocked network, thus 
providing a false positive. One route Google Chrome uses to determine network
connectivity is `http://clients3.google.com/generate_204` [docs][1].

[1]: http://www.chromium.org/chromium-os/chromiumos-design-docs/network-portal-detection