Add default option for Slack + fix avatar
Fix settings.json

 - Add comma to fix json valdiation
 - Rename channel1 & channel2 parameters to ned1 & ned2 to avoid
"undefined" parameter to ffmpeg in stream.js
Use settings.port for server
Unfurl url for slack

See "Automatic unfurling" here: https://api.slack.com/docs/attachments
Merge pull request #3 from j0k3r/slack

Some Slack improvement & fix in settings.json