*.supplement files are typically *.zip

[amussey]

Certain books include a supplement "version". For example:

In this case, it includes some runnable code samples that correlate to those in the book. humblebundle-ebook-downloader --format all handles fetching of these files perfectly. However, they are assigned the extension *.supplement.

In my limited experience (6 books in my library with supplements), these are downloaded from the Humble Bundle site as *.zip files. Rather than My Book.supplement, could we have these downloaded as My Book (supplement).zip?

(Thanks for the handy tool!)