Simple command line interface for ensuring environment variables are defined.
This is meant to be used with npm scripts when environment variables are required for scripts.
$ defenv [ENVIRONMENT_VARIABLE]
When the given environment variable is properly defined, the script outputs nothing.
$ defenv PATH
$
But when you've given the script a missing environment variable, an error is issued and the process exits with an error.
$ defenv AWS_SECRET_KEY
**AWS_SECRET_KEY** is not defined