Add "required setting"

[BasicWolf]

There should be a way to indicate that a setting must have a non-Undefined value.

It can be easily implemented as validator, and a shortcut class:

class AppSettings(Settings):
    KEY = Required # Should call Required.inject(setting)?
    KEY = RequiredSetting()  # no initial value in __init__()!
    KEY = required(Setting()) # exception if initial value is not Undefined
    KEY = Setting(validators=(..., is_defined)) # basically the two above can do this!

After implementing behaviors, the following is possible:

class AppSettings(Settings):
    KEY = Undefined @ required

[BasicWolf]

Implemented in #11