Use the plugin API to the getter and setters

To avoid having to redefine these methods on the deprecated plugin we should be using the `sanitizer_vendor` API.

cc @kaspth

PS: I think we should undeprecate the `:protocol_separator`, `:uri_attributes`, `:bad_tags`, `:allowed_css_properties`, `:allowed_css_keywords`, `:shorthand_css_properties`, `:allowed_protocols` options on the deprecated gem right?