* Bing works again
* Removed Sogou, Naver and Yahoo! Japan
* Manifest v3

Update Naver & archive.today template urls

If you don't see any template url changes, go to the options page, delete rows for Naver/archive.is, press Merge & Save.

* Add Merge btn to the options page
* Bug fixes

If you don't see new cache providers, go to the options page, press Merge & Save.

* add more cache providers: sogou, naver, yahoo jp, time travel
* add a spinner for context menu clicks
* auto close the popup window after a click
* minimum required FF version is 60.4.0

if you don't see new cache providers, go to options & press 'Reset' btn

* Display a context menu for text selections & images
* Fix a breakage under FF 63+
* Fix an alert() dialog from the background page

* (breaking) Switch to the async chrome.storage.sync instead of window.localStorage. This means you'll have to reenter your custom providers (sorry about that).
* Options UI: switch to the 'native' html5 dialogs instead of alert/confirm.
* Fix several cases where the context menu didn't update properly.