Add extra options to doc configuration

[elbrujohalcon]

As described in inaka/sumo_db_mysql#6, schema names are by design equal to module names for the modules that implement sumo_doc behaviour.
That's an undesired limitation that we should avoid by adding an optional extra parameter to each docs configuration where these kinds of things and others can be specified.
That way, in situations like the one @jordan0day describes, instead of…

    , { docs
      , [ {user, the_store}
        , …
        ]
      }

…we would have…

    , { docs
      , [ {user, the_store, #{module => my_users}}
        , …
        ]
      }

…or…

    , { docs
      , [ {my_users, the_store, #{schema_name => "user"}}
        , …
        ]
      }

[elbrujohalcon]

After this feature is implemented here, we will need to adjust all the drivers to support it.