Markdown cells could support <br> as line breaks

[merplumander]

📝 Summary

I wish I could enforce a line break in markdown cells with something other than two spaces at the end of the line. Trailing spaces are removed by many automatic code clean-up procedures such as black and so I consistently have the problem that my markdown cells end up without line breaks.

🏁 Motivation

My hope is that I can keep using black to automatically format my .py files. While also having the possibility of having line breaks in markdown cells in these .py files.