Enable rotation of more options than just secret token

[caius]

You might want to be able to change other options to ActiveSupport::MessageVerifier (for instance, change your serialiser from Marshal to JSON, or digest from SHA1 to SHA512.)

Might be nice to support different verifiers on-the-fly, so you have a deprecated one that unserialises with Marshal, whilst the current active one uses JSON, to migrate from Marshal to JSON on-the-fly without logging everyone out.