Make output filenames user-configurable

[mkazin]

Other users of this utility will have their own particular preference when it comes to file naming.
Some might even want to use a shared configuration file.
Some of them may not know how to- (or want to) code to get it working the way they like.
I'm thinking of something similar to standard date formatting.

There may be some use cases to consider. For example:

The Vanguard extractor supports Quarterly statements:

    def rename(self, extracted_data):
        return self.__class__.FILE_FORMAT.format(
            extracted_data.get_end_date().year,
            extracted_data.get_end_date().month // 3)