Automatically hide local sites (e.g. "about:" & "file:").

Addresses #15. I would prefer to make it configurable, but not sure of the best way to go about it. For new users, these sites could be the default hidden list, but that's problematic for existing users (if it's blank is that because they don't want to hide anything, or haven't hidden anything and these sites should be added?; if it's not blank, do we add these, or have they already been added?). Perhaps another edit field for local sites might be the go? Or just always leave them hidden.