* Run script at document end
This way, we don't have to wait for things like images and frames to load before running. See: https://developer.chrome.com/extensions/content_scripts#run_at
* Leverage ES6's `const` for values we know won't change
* Simplify conditional logic
* Instead of passing `null` around, just throw an error when we ecounter one
Then we can catch the error and log it
* Remove `setInterval` code, since Chrome injects the script after the DOM is loaded
* Remove chrome from global
* Break up const declarations into multiple statements
* Use traditional comparison syntax
