Add Slack client

[JeanMertz]

At our company, we have a lot of Slack bots and integrations. It would be nice if Automaat was one of those tools that integrates with it. This way, you don't have to open your browser if you are active on Slack, and need some information or perform some action that Automaat can provide to you.

In essence, this means writing a small proxy between the Slack API and the Automaat API.

It would be fairly similar to #50 in that you can search for tasks, and run them with the right variables.

This doesn't have to live in the main repo (just to keep things are bit more compartmentalised).