add the ability to sell or replace roles

[iMxAndyy]

• Cog: buyrole
• Issue description: if someone buys more than one the one that is displayed changes depending on its hierarchy in the server which may not be what the user wants.

Could you add a !sellrole command to sell a role back for a set cost.
and/or a toggle to only allow people to buy a total of 1 role.
meaning remove from previously bought roles before adding them to their new role if the toggle is true. Although I realize this would mean creating a log of purchases to undo but it would make this cog more versatile.

Thanks