Allow throttle period option to be a proc

I need to filter requests on a limit and period I get dynamically out of a request environment. Currently, I can work out the limit, as it can be a `Proc`, however I can't do it for the period.

This PR adds support for that. Tried to do it in a way that doesn't brake backwards compatibility, as periods are coerced to numbers during `Rack::Throttle` initialisation.