Enable using different key-casing

[jendakol]

It'd be great if loading from Config supports multiple possible casings - currently only camel case is supported. IMHO ideal would be to be compatible with set supported by Pureconfig: https://pureconfig.github.io/docs/overriding-behavior-for-case-classes.html#field-mappings
At this moment, I see no support for such thing in this library; however I believe it could be implemented in a reasonable time and I'm willing to try it - would you be interested or I should do it as my personal project?