Slack notification mentions

[tkupek]

I implemented the notification today and it works perfectly. Thanks for this piece of code 👏

One thing to mention: In order to make the Slack notification work with mentioning users, one needs to either use the user-ID (<@XXXXXXXX>) or enable link_names=1,.
It seems like the API has changed in 2019.

https://stackoverflow.com/questions/40771924/how-to-mention-user-in-slack-client

[twsl]

I think, using names instead of user ids should be the default, as it enables mentioning channels too.
I'll fix this soon unless you want to provide a PR

[tkupek]

The argument from Stackoverflow is that link_names does not work when mentioning multiple users. But I guess it does not hurt to enable it anyways: #11