Fix adblock plugin with new request plugin API
Add support for optional plugins

Plugins that mark themselves as optional (via config file) can
be enabled or disabled via the 'X-Gonzales-Options' header sent
by the client. The form is "+foo -bar" to enable plugin 'foo' and
disable plugin 'bar'.
Make the default plugin config disabled

Added and enabled all of the previously implicitly-enabled plugins
Don't tunnel HTTPS traffic by default