Unifying switches and config options.

There was a rule for command line switches, and for ini file options,
where in command line options words are separated with hyphen and in ini
files options are separated with underscore. This rule wasn't always
enforced, fixed it now.

slack_backup/command.py

@@ -96,7 +96,7 @@ def main():
                        help='Use specific config file.')
     fetch.add_argument('-r', '--raw-dir', default=None,
                        help='Write raw responses to provided directory.')
-    fetch.add_argument('-f', '--url_file_to_attachment', default=False,
+    fetch.add_argument('-f', '--url-file-to-attachment', default=False,
                        action='store_true',
                        help='Treat shared files (but not uploaded to the '
                        'Slack servers) as attachment. By default there will '