Make Containers support optional

[peter-lyons-kehl]

Please consider making Containers support #24 optional.

This would need two things:

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_permissions (https://support.mozilla.org/en-US/kb/manage-optional-permissions-extensions) and
• a preference/option as to whether handle/support containers or not. That would control/activate functionality from (feat): support firefox containers #24 (457a1c2).

How could we treat existing users and their expectations? Is there any way to make this new option disabled by default for new installation, BUT enabled by default for existing installations (that is, for installations where we are updating a version)?
If too complicated, we could enable Container support by default. If the users wants, she/he can uncheck the option and remove the optional permission.

[peter-lyons-kehl]

The current problem: copy-selected-tabs-to-clipboard enforces Containers:

[piroor]

Sadly "contextualIdentities" permission couldn't become optional. Only permissions listed here can become optional.
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_permissions#api_permissions