User defined cache key generator

[atoncetti]

Adds the possibility to change the cache key with a new option field cache_key_generator.

This makes it possible to cache two requests to the same URL http://foo.com/?q=1 based on other information in the request, for example, different headers.

Use case example: caching requests to the same URL but performed by different users (different authentication headers).

Other changes:

• Update deps
• Add formatter support
• Improve testing setup