Add cascading defaults as an IniDefault option

[crustymonkey]

This adds an option, cascade_defaults, that can be set to enable the cascading of values in the [DEFAULT] section to other "concrete" sections. This is NOT enabled by default to preserve backwards compatibility. When cascade_defaults is set to true, this will mimic the behavior of the Python configparser library, wherein the default value will be returned for a section/key lookup that doesn't exist. For example, with this config:

[DEFAULT]
fallback_key = fallback value

[mysection]
other_key = other value

With cascade_defaults set to true, a config.get("mysection", "fallback_key") call would return Some("fallback_value") instead of None.

I've added unit tests in the test.rs file to cover the new behavior. I've also attempted to generally follow the same styles used in code/comments.

[crustymonkey]

Ok, looks like the default workflow died due to a lack of cargo fmt. Ran that and pushed a commit for that.

[crustymonkey]

Ok, attempt #2 of cargo fmt with standard max_width

[QEDK]

@crustymonkey thanks for your contribution, merged!

[crustymonkey]

Thanks for merging. Sorry about the churn with the formatting. I had forgotten that I made a global max_width change for rustfmt a while back.

[QEDK]

Thanks for merging. Sorry about the churn with the formatting. I had forgotten that I made a global max_width change for rustfmt a while back.

no worries, we appreciate OSS contributions and mistakes naturally happen :)