Allow turning off the extension

[elis]

I love the extension, but one thing that really bothers me is that I can't turn it off during day time.

On chrome I've made the extension to be active only when clicking the icon - but that's kind of annoying in itself as it needs to be activated on every tab I'm using github where I want the theme to be dark, and if I open a new github tab it's back to being disabled (and light theme) until I click the extension icon again.

On firefox I don't even have that, it's either installed or removed, ignoring the whitelist/blacklist option in the extension itself.

Please, just add a simple toggle button that will allow me to globally enable or disable the functionality of the extension at will.

[Sottti]

Related to #159, #148, #107, #32