Please consider adding SkipList option

[vkuznet]

Hi, this is a very nice of software we'd like to use in our stack but it lacks of SkipList option which is very useful to allow certain clients to bypass limiter functionality. Would you be willing to consider to add this functionality? I need to stress out that in combination of custom HTTP header to use it will significantly increase its usage. For instance, the limiter can be applied to a custom HTTP header which identifies user, e.g. at CERN we use user's X509 DN. As such we can apply this to GRID jobs and limit users (abuser) based on their DN. But for production services we want to avoid that and can put their DNs in skip list if it will be provided.

[vkuznet]

I provided a prototype of changes in this PR #186. I'm open to any implementations and only would like to stress out that such addition would be very useful for our use case.