New header row added to CSV each time the script is run

[DanOBrien]

We only need to write the headers once, so if we modify the "write" block to see if this is the first time we're writing the CSV, we don't keep seeing headers added to the file when we've gone back to add new books:

if len(new_additions) > 0:
    with open(filename, 'a') as csvfile:
        writer = csv.DictWriter(csvfile, fieldnames=fields)
        if current_ID == 1:
            writer.writeheader()
        for item in new_additions:
            writer.writerow(item)