[Enhancement] SimplePolicy: Reasons for Instance-specific Policies

Mostly since I feel like this would make some Unlisting/Reject/Quarantine policies a bit clearer if they allow for some form of optional description to be displayed under Federation at /about

The only way I could really think of adding arrays into the listing in MRF.exs to be parsed as ["instance","reason"]

Seeing that the priority on a enhancement like this would be rather low, I might see if I can work on implementing this myself and if I get it to work I'll prolly PR it. But chances are low since I am not all too comfortable with most of Elixir.