Add metadata on Slack unfurling

Slack unfurls links [1], displaying some metadata about the linked site
in the chat window. This <meta> elements will allow to display a preview
on when a chist link is pasted into Slack.

[1] https://api.slack.com/docs/unfurling
Merge pull request #74 from threefunkymonkeys/feature/slack-unfurling

Feature: Slack unfurling