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}]
```
With the name I wanted to emphasize that it is not an object that represents a TokenBucket, but rather a set of parameters to create one. On the other it is one of the few things that represent the package interface, and will be accessed using package name. `tokenbucket.Config` how about that?