Added raw-dir option

This option can be used to debug slack API responses - if set,
slack-backup will dump all of the data as a JSON in provided direcotry.

slack_backup/command.py

@@ -94,6 +94,8 @@ def main():
                        help='Path to the database file.')
     fetch.add_argument('-i', '--config', default=None,
                        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,
                        action='store_true',
                        help='Treat shared files (but not uploaded to the '