Add FeatureRuntimeAttributes

In my company, we are starting to use togglz on backend application with no user and we need to be able to activate/deactive features depending on runtime attributes.
To be more precise, we are processing requests in Camel and we will want to have the feature active depending on data which is part of the request.

For that, we are proposing to introduce  FeatureRuntimeAttributes which are attributes transmitted to the ActivationStrategy directly from the feature.

In our case, we envision to pass directly the data from the place where we want to check for the feature activation.

I might need to add some tests to show more in detail how we think about it and see with you if this could be mergeable or the changes you would like.

Thank you
Fabien