Feature request: mark certain config keys as required

Is it in scope for Donfig to mark certain config keys as required? My use case is one where I'd like to require a user to specify some keys as part of their local config (like how Git requires you to configure your user name and email).

Two potential APIs that would work for my use case:

1. Insert a sentinel value into defaults

from donfig import Config, required  # sentinel value

config = Config(
    'my_lib',
    defaults = [{
        'user': {'name': required, 'name': required},
        'foo': {'bar': None},
    }]
)

2. A separate constructor arg for required keys:

config = Config(
    'my_lib',
    defaults = [{
        'user': {},
        'foo': {'bar': None},
    }],
    required=['user.name', 'user.email']
)

I'd like to raise an error if a user required value is not found in any of the user configs (or environment variables) during setup of the Config class.

Start by reading the Config class constructor and its setup path, focusing on how defaults, user configs, and environment variables are loaded. Compare the two proposed required-key APIs and define how missing values should be detected and reported during setup.