Refactor main.py config hardcoding

[dannylty]

Find a way not to hardcode the indexers and writers?

    stream_indexers = (HolodexIndexer(), NijisanjiIndexer())
    writers = []
    if config_handler.write_to_db:
        writers.append(DatabaseWriter(config_handler, None))
    if config_handler.write_to_local:
        for folder in ['simple', 'metadata']:
            os.mkdir(os.path.join(config_handler.local_path, folder))
        writers.append(FilesystemWriter(config_handler, None))