Support for Extreme Limits

[dschanoeh]

Hi,

Thank you very much for this nice library. It fits my use case nicely and allows to apply lots of different restrictions at the same time.
One small gap I see is that it doesn't seem to be possible to apply the extremes of "unlimited" or "completely blocked" when using LimitFunc.
Consider the following usage:

func byCustomerID(customerID int) int {
    return customerID
}

func getCustomerLimit(customerID int) Limit {
    plan := lookupCustomerPlan(customerID)
    if plan.Terminated {
        return rate.Blocked
    }
    if plan.Unlimited {
        return rate.Unlimited
    }
    return plan.Limit
}

limiter := rate.NewLimiterFunc(byCustomerID, getCustomerLimit)

For "unlimited", one could use a workaround with extremely high counts but this'd consume unnecessary memory and is a bit inelegant.
For "completely blocked", one could naively try something like NewLimit(0, time.Second) but this leads to a division by zero.
Therefore, I think it'd be useful to have special limits representing these extremes and allowing to effectively skip the regular rate limit processing when encountered.