New feature: custom method to fetch env

[emilien-puget]

Hello, would you be interested in having a way to register a function to fetch a value from somewhere

i imagine this with the ability to have an option to change the method now called getOr, with the current implementation being the default one.

My present use case is to fetch informations from a vault-like external service

[caarlos0]

hey, @emilien-puget , thanks for the suggestion, but I really like to keep the lib simple, so, rather not add this unless we really, really need to.