Open diff viewer directly from quick access button

[NonSwag]

It would improve the user experience and speed up workflows if clicking the quick access button would directly open the diff viewer. Right now, the extra step slows things down a bit

Not sure if the quick access button has a name, but I mean this one

[jpenilla]

Currently the extension code is 100% shared between Firefox and Chrome, because the APIs are the same for popups. Adding actions directly to the button requires background scripts which have different manifest entries for each browser. This would require some build setup.

I'm not sure whether I prefer

• Click icon for current behavior, shift click for instant open
  or
• Click icon for instant open, add an options page to configure the URL

But generally I'm open to these kinds of changes even if it requires some build ceremony.

[jpenilla]

The extension now has an options page, now just need to decide whether to keep the popup and add a shift-click action or get rid of the popup.