Unify configuration

[teamon]

Currently there is some inconsistency regarding adapter/middleware configuration.
For example, logger format can be configured with config :tesla, Tesla.Middleware.Logger, debug: false but log_level can't.

This all should be unified with clear rules of precedence.
Also relates to #219

[teamon]

Global config override can be useful for cases like changing log level in test:

config :tesla, Tesla.Middleware.Logger, log_level: fn _ -> :info end

(This is currently NOT possible)