Allow multiple subscription search criteria

[EPICmynamesBG]

when using /subscribe, allow multiple criteria to be entered as an OR condition
ie: /subscribe lobster, chowder, beef

optionally, allow another /subscribe with a search term to append/update the current, rather than forcing users to unsubscribe and resubscribe. This may be better done with Slack app status and using response buttons
ie. /subscribe chicken, /subscribe lobster will return 'Replace Subscription' or 'Add to Subscription' type buttons