Currently, if message contain shared file, slack-backup will try to
download it. If it fail, than empty file will remain, which will be at
least confusing. This will mostly happen for shares which are not
uploaded to the slack servers.
New option will be used to indicate if slack-backup should convert such
share as an attachment, or to save the list of URL and their
destination in local file system to be download manually by the user.
For some reason, database key was treated differently in configparser object
in python 3.4.2. In Python 3.4.5 everything is fine. Fixed the defaults to
make sure all string options are treated equally.
