Add cookie option "partitioned" to DEFAULT_OPTIONS and documentation

[womblep]

Add clarity for Issue #42 by documenting that partitioned can be passed when setting up Persisted sessions. Sets a default of false to partitioned in DEFAULT_OPTIONS. Will only be active once Rack 3.1 is deployed which adds the partitioned attribute to cookies in Response.set_cookie

[j05h]

👍 thanks for this @womblep. Was just looking for this!