Include current environment variables for child processess

[vkareh]

When using this module in Mac/Windows with a node app that spawns child processes, those processes lose all their environment variables unless passed explicitly.
Extending over NODE_ENV also ensures the proper value in case a user has it defined as a system-wide environment variable.

[narirou]

Hi, @vkareh.
This problem should be fixed and environment values should be inherited as a default.
So I would like to implement it inside of a function.

Thanks.