Add support to middlewares

[nettofarah]

This is something @timhudson and I have been discussing.

By adding support to middlewares/interceptors we could easily extend jsonrpc2 to support some extra functionalities.

Some features that come to mind:

• Better logging/reporting: We can write specific logging/reporting functionality in the clients themselves.
• Support for fixtures/vcr so we can use mocked servers in test environments
• Implement client specific retry logic
• Dynamic timeouts
• Circuit breaking