Extension to justified-containers

[L0neGamer]

Hi, I made a start on a more wide-ranging justified-containers approach, which you can see here: https://github.com/L0neGamer/justified-mono-traversable. The key difference is that this relies on mono-traversable, and can be used to define an instance of the justified methods as long as the correct mono-traversable instances are defined.

Please let me know if this is something you're interested in for justified-containers.