added command line arguments for 1. timeout length 2. number of retri…

[chopley]

I've added the following:

1. --timeout -- allows the Slacker timeout argument to be passed to support downloading data from longer slack conversations
2. --retries -- allows the Slacker rate_limit_retries argument to be passed
3. -- regex -- allows a regular expression to be given to filter chosen groups or channels rather than having to pass an explicit list.