Add --sleep-interval option to sleep between downloads during a batch process

Adds the option --sleep-interval <seconds> (default 0)

Waits the specified number of seconds before downloading the next file in a batch.  This is helpful for dealing with "503: Service Unavailable" errors that sometimes occur if one download is quickly started after the previous.  I've seen this happen sometimes with vimeo channel downloads.
This should only be printed when sleep interval is provided. It's kind of misleading and unrelevant to see when you didn't want any sleep interval.