acai: Pass external logger as a function to the library

[MartinHelmut]

To provide logger functionality it should be possible to pass a logger instance as option to the library. The callback interface could look like the following (written in TypeScript):

enum LogLevel { Info, Warning, Error };
type loggerCallback = (message: string, level: LogLevel) => void;

Usage could look like:

(async () => {
    await scanner('path/to/repo', {
        log(message, level) {
            // map logger to callback ...
        }
    });
})();