Allow override configuration for specific read calls.

[javierbrea]

In some scenarios could be desirable to modify configuration only for certain invocations to the "read" method. (For example, when passing an "idempotency-key" as an http header using "mercury-api")

This options could be passed as a parameter to the "read" method, and should override instance options only for that execution. For other methods, such as "create" or "update", options could be passed as second argument.