env: add a 'convert_case' field to ease dealing with kebab-case

[da-x]

This allows usage of kebab-case attribute in serde, mapping
unambiguously into a config value given a multiple character separator.

This also add the convert-case feature.

For example:

let environment = Environment::default()
    .prefix("PREFIX")
    .translate_key(Case::Kebab)
    .separator("__");