Port to WebExtension #60

[brunoais]

https://bugzilla.mozilla.org/show_bug.cgi?id=1423725 is now on Nightly.
Are you able to resume your work on this extension? If not, what's missing?

[denschub]

I said in #60 (comment) that I will update the issue when I have something to update. I also said I will port the extension as soon as all required APIs are implemented and I had the time to actually port it. And all these statements are still valid.

I am not a wizard.