[Feature Request] Add functionality to read enviroment variables

[jorickert]

Hello,
first thank you for this great project!

Something that i would like to see is an function to read enviroment variables.
It should be able to tell me if a variable is set and if it is set its value as string.
My use for this would be to access things like build-id from a CI system.

Thank you

[nirbheek]

Duplicate of #9.