Implement __getitem__ and __setitem__ methods

[ziggycross]

It seems the current approach for managing config is with .get and .set methods.
I believe it would make sense to also implement __getitem__ and __setitem__ dunder methods in addition to this.

For example, the code example

config.set('number', 42)
config.get('number')

could also be written as

config["number"] = 42
config["number"]

Looking at the source, this should be a relatively small update to implement. If you think this would be acceptable, I am happy to add it myself and submit a PR.