Feature: Slack notifications

[pawroman]

As a feature, we could support Slack notifications when link checking fails.

This could be done as a webhook or alternatively, using ready-made GitHub actions or other integrations. In this case the README could include example of how to enable this integration.

[mre]

If someone wants to give this a try, there are Github actions like these that we could use: https://github.com/marketplace/actions/slack-notify. I'd prefer that over bundling the Slack integration into lychee-action, because it would be more flexible to configure and it would be easier to stay up-to-date with Slack API changes.

Steps to help out here:

1. Add lychee and Slack action to a test pipeline
2. Try a test run to see if it works (e.g. it sends a Slack message on lychee failure)
3. Send a pull request which adds a usage example to the README.md