Have Arkana support SCREAMING_SNAKE_CASE env var names

[rogerluan]

Currently the only supported case is CamelCase due to how the mapping is done internally within Arkana. However, it's pretty standard to have SCREAMING_SNAKE_CASE being used as the capitalization rule for env vars (in any programming language that I know of). For this reason, we should either make screaming snake case an alternative and valid case, OR make it the only valid case, OR allow for custom mappings (e.g. a file that maps between the env var name and the actual env var identifier it should map to when generating the code in e.g. Swift, Kotlin, etc), for instance:

{
    "API_KEY": "apiKey",
    "AUTH_TOKEN_KEY": "authToken"
    …etc.
}

[davemess]

+1