Global, user-specified, @exclude rules

[arantius]

Originally: DevjaVu ticket 18

The end user should have the option to define a set of global @exclude rules that would completely disable all Greasemonkey scripts, when matched. Adding this functionality would need:

• A place in the UI for this.
• Some sort of observer that would update the enabled/disabled icon when the user navigates to, or switches tabs to, an excluded page.
• (Optional) A quick "Exclude this domain" item in the Monkey Menu to make this feature more discoverable. If we allow full @exclude syntax, this is harder to make reversible.