Unassign permissionsets from users when removed from source tracking

[geoffswift]

So imagine this:

• I'm using source tracking
• A permission set is removed from my repository

When I try to do a sf project deploy start, I'm greeted with the error:

This permission set is assigned to one or more users.

I then need to unassign the permission set from all users before I can then deploy successfully. It'd make life easier if the cli could cope with this, and it would "just work"