Add substitute environment variables feature.

Hi there,

I wonder if it is possible to add usage of environmental variables in a config file? The thing is I rely on the env variable that has a changeable value. 

I'd like to be able to use the style similar to the following one:  

my.env=${MY_ENV}

As I understand TypeSafe config has this feature.

Best regards,

Dmytro Zhydchenko.
