Feature Request: Add prefered file format for ebooks

[marcstraube]

First of all: Thank you for your work! It makes downloading my stuff from HumbleBundle a charm.

There is one feature I'd like to see in the future: Let me define a list of prefered formats for my Ebooks and only download the first matched file type. I have a lot of comics in my purchases and most of them come in three different formats (cbz, epub, pdf). It would be nice to define a list of formats like this:

ebook_format: cbz epub pdf *