Account per world

[SOF3]

As requested by @reyyan987 in poggit-orphanage/EconomyS#76

This should involve creating a schema called "world", with the following config:

schema:
  '#type': The type of schema. Possible values are "basic", "currency", "world".
  type: world
  '##': In the world schema, each player has one account for each world.

There are several possible configurations, but I am not sure if they are actually useful:

• World groups: Worlds are grouped such that player uses the same account in the worlds in the same group
• World whitelist/blacklist: Player cannot use any accounts at all in these worlds

[SOF3]

It is not implemented, but it should be very easy to add support for this. In fact, the Schema interface is designed to support the World use case, which is why I lately worked on lazy account creation.

[SOF3]

@reyyan987 See #35