support Chrome via WebExtension browser API Polyfill

[gianpaj]

I followed the instructions from the Mozilla repo: https://github.com/mozilla/webextension-polyfill

• [x] Make the CSS injection work
• [x] Make the JS execute
• [ ] Test on Hackernews all the CSS and JS
• [x] ~Test on Twitter all the CSS (broken/outdated at the moment)~
• [ ] Test on YouTube all the CSS and JS
• [ ] Test on Reddit all the CSS and JS
• [ ] Test on YouTube Gaming all the CSS
• [ ] Test on YouTube Music all the CSS and JS
• [ ] Test that the extension does nothing and causes no errors in an unsupported site