Make the api config runtime-configurable

[tstenner]

• replace the map-based INI reader with a callback based function
• move default values to the variable initializers, i.e. a default-construct api_config has sane defaults
• add function api_config::set_option(), could be exported to the C API in the future
• add optional bool parameter to skip loading settings from a file (needed for the unit test right now)
• throw errors on some invalid settings