Bug #3547: Block all plugins except flash.

We cannot use the @mozilla.org/extensions/blocklist;1 service, because we
actually want to stop plugins from ever entering the browser's process space
and/or executing code (for example, AV plugins that collect statistics/analyse
urls, magical toolbars that phone home or "help" the user, skype buttons that
ruin our day, and censorship filters). Hence we rolled our own.

See https://trac.torproject.org/projects/tor/ticket/3547#comment:6 for musings
on a better way. Until then, it is delta-darwinism for us.