Add a Slack handler that communicates with the Slack API

[otech47]

Related to #55

• Integrate the SetLife Slack app with proper credentials
• Run a basic test that lists all the channels in the SetLife Slack

Example implementation:
https://github.com/HackerFund/cic-slackbot/tree/ol/issue-2
HackerFund/cic-slackbot#5