Fix gif2video with new request plugin API
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
Merge branch 'optional-plugins' into develop