Option to enable/disable by default

Describe the issue

I've installed fresh version. And the extension is on by default for all sites.

Could you please add option to set On/Off by default for all sites? I just want to turn it on for some sites. I don't want to manually turn it off for every site I visit.

Thank you,

[adisib]

Read these.

https://github.com/gorhill/uBlock/blob/master/CONTRIBUTING.md
https://github.com/gorhill/uBlock/wiki/Dynamic-filtering:-turn-off-uBlock-everywhere-except

[gorhill]

Duplicate of #1623.