Makes keyword arguments usage compatible w/ Ruby 3
Makes keyword arguments usage compatible w/ Ruby 3

In Ruby 3 keyword arguments are completely *decoupled* from positional arguments (see e.g. [ruby-lang.org News](https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/)).

This fix makes the LinkFormatter compatible again with the new semantics of keyword arguments in Ruby 3.
stevenosloan/slack-notifier