Books with ? characters

[cdlvcdlv]

I had a book which name somehow had '?' characters in it, causing the download to finish with an error in line 142 of main.py.

I fixed it changing line 209 in main.py from

book_name = book['productName'].replace(' ', '_').replace('.', '_').replace(':', '_').replace('/','')
to
book_name = book['productName'].replace(' ', '_').replace('.', '_').replace(':', '_').replace('/','').replace('?','_')