allow exceptions per site / page / page elements

[aspiers]

The extension mechanism automatically gives users the choice of whether it's active on all sites or just selected ones, but probably it would be helpful to allow more fine-grained exclusions from this, e.g.

• Allow on all sites by default, but exclude selected sites / pages matching any regexp in a list.
  (This is kind of the opposite of the "enable per site" approach mentioned in Operate on limited sites by default, but allowing expansion #21.)
• On selected sites / pages matching any regexp in another list, exclude page elements matching any item in a list of CSS selectors (or perhaps even XPath selectors)