Provide immutable config objects

[joerunde]

Is your feature request related to a problem? Please describe.

Currently all aconfig.Config objects are mutable.
When I have a config object that I want to distribute across an application, my choices are either:

• Trust that developers never write code that modifies this config object, or
• Hand out a full copy of my config so that any code accessing it is guaranteed not to muck with anything if it makes changes to the config

Neither of these choices are all that great.

Describe the solution you'd like

If a config can be created with an option to be immutable, such as:

mutable_config = aconfig.Config({1: 2, 3: 4})
immutable_config = aconfig.Config(mutable_config, frozen=True)

Then I can pass around the immutable_config to any piece of code that wants it without worry, or expensive copying

This can probably be implemented by swapping MappingProxys in the guts of the config code instead of plain dicts

Describe alternatives you've considered

Not implementing this and crying?

Could maybe slap some code in __setattr__ that checks if the thing is supposed to be immutable and stops any mutations.
That sounds a little less robust than the proxies which have no __setattr__ at all.

Additional context

Add any other context about the feature request here.