Filter for 'didn't unfurl' messages in slack output.
Filter for 'didn't unfurl' messages in slack output.

If Slack sees the same URL frequently, it won't unfurl them.
When using Direct Messaging to the bot, this will cause the
"Didn't unfurl..." message to be sent to Watson conversation,
and will cause a cascade of errors.

Closes: #107

IBM/watson-online-store,IBM/watson-online-store,IBM/watson-online-store