Add feed that has countdown

[kevinsimper]

Christopher from HackYourFuture want to post to Slack when there is 7 days to CopenhagenJS.

He is using Zapier to post on Slack and Zapier can read from a ATOM feed.

We should add a feed that can be customized when to add a meetup to the feed.

/feed.xml?daysbefore=7 Will add the event 7 days before it happens

/feed.xml?daysbefore=2 Will add the event 2 days before it happens

😄