add 'Disable everywhere' option

[santosoj]

For a way of toggling the extension globally that's more convenient than disabling and re-enabling it in the browser's Manage Your Extensions view, a Disable everywhere option is added to TransOver. It works in the same way as Disable on this site, except that it toggles the extension for all sites.

[artemave]

I'll merge this - it's easier than reviewing pr.