withAccessToken: allow dynamic tokens

https://github.com/mittwald/api-client-js/pull/45 introduced the common interceptor withAccessToken.

If changing the access-token at runtime should be supported (what I suppose is a real-world-use-case), the withAccessToken interceptor is not the right solution, because it would require to create a new client and replace all usages.

Maybe the token parameter could be a function (executed in the interceptor) or a variable bag { token: string } what can be mutated during runtime – but this is a not so common and reasonable approach.