Add support for Opera

[Croydon]

It might be relatively easy to support Opera as well.

Blocked by:

• [Meta] Refactor to WebExtension add-on #13
• Replace all -moz- CSS attributes #87
• https://bugzilla.mozilla.org/show_bug.cgi?id=1404746

[Croydon]

I will support Opera in the future.

I got it this far in like 10 minutes:

[Croydon]

https://bugzilla.mozilla.org/show_bug.cgi?id=1404746

[Croydon]

Opera is not willing to extend their WebExtension APIs with anything which they don't get via their Chromium base and I hoped they would.

Too much functionality VTR is using is Firefox-only and I'm not willing to put that much time into trying to find workarounds for a closed source browser.