Add option for specifying the path to the .example file

[venikman]

I need to support multiple environments for my server. Also different environments have different variables. But there is not a clean/simple way to do it.
Right now to support 2 envs you will have to create .env.prod and .env.prod.example pairs for each env.

Ideally I would like to have max to files but maybe when you call:

    const env = envy({yourEnv});

and inside .env.example and .env you can have few functions???