Add Kibble Scanner for Slack

[sharanf]

Description
Kibble doesn't currently include a scanner for Slack and a request for one has been made.
Create a new kibble scanner to accept Slack as a data source

Use case
Kibble should try to cover as many of the the popular sources as possible so that users who prefer using Slack may
be able to scan and report on their data

Related Issues
Original Request [https://github.com/apache/kibble/issues/87#issue-730783106]

[michalslowikowski00]

Hey @sharanf.
I would like to work on that. Are there any requirements or any other information that could help? :)

[turbaszek]

Are there any requirements

Probably would be good to merge #78. Then we can also start writing tests and run them on CI to avoid regression