An optional environment variable based decoder for goschtalt.
This decoder provides both an example of using the goschtalt.AddValueGetter() option
as well as a simple way to read in structured environment variables.
While this approach might seem like a great path, a far better way to interact
with environment variables is by creating a configuration file & using the built
in substitution approach via ExpandEnv(). The reason for this is environment
variables often have limitations on the key case that may make their usage hard
on some systems, or across systems.