Make tokenlimiter capable of getting rates from a request header

Ratelimiting middleware can be configured to retrieve Rates to be applied to requests. To do that one needs to pass a **ConfigMapper** function in an **Options** struct to the **NewTokenLimiterFromOptions**. The function should map an http request to a JSON string that configures token buckets for it. There can be configured several token buckets for a request. A request will be allowed by the middleware if all token bucket allow its consumption. The JSON token bucket config should look like this:
```json
[{"PeriodSec": 1, "Average": 10, "Burst": 20},
 {"PeriodSec": 60, "Average": 100}]
```
```Go
rs = tokenbucket.NewRateSet()
rs.Add(time.Second, 1, 1)

rs[time.Second] = nil // Oops the rs value is no more valid
```
The last line show that user of RateSet can modify the state. Of course the user should be either malicious or stupid to do that, but I prefer to be on the safe side. And do not expose more than necessary.