Links to warehouse bundles should unfurl in Slack

[vanstee]

If someone drops a link to a warehouse bundle in chat, Slack should be able to pull down metadata about that bundle and show a message attachment about it. Slack calls this unfurling and has plenty of docs on the topic: https://api.slack.com/docs/message-attachments

We should probably include the basics of the bundle and also include the command to install it if possible. Something like @cog bundle install cfn if they linked to https://bundles.operable.io/bundles/cfn