Using url_file_attachment user can specify if he like to change objects
like 'file_share' marked as external (in Slack servers point of view)
to attachments, so the only value would be remembered URL for the
"uploaded" document as an attachment. Or, treat it as is, and let the
slack-backup to produce file which contain the URLs and corresponding
local file names for such files.
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.
