Options page now respects light/dark themes

Starting in v2.0.0 the default behavior changes to a per-site mode. This greatly improves privacy by reducing browser fingerprint uniqueness. As a result, after upgrading from 1.x, you will need to go to each site that you want to have PageSpeed disabled for and toggle the extension state for that site.

Made the wording on the toolbar button tooltip/label more clearly indicate the current state.