Provide a default value for NODE_ENV

[gvilarino]

Since the module defaults the environment being used to development when there's no NODE_ENV present, for coherence's sake we should provide that value as part of the resulting config object (as a default) either under nodeEnv, environment or env.

Bottom line: we should expose the environment name being loaded by the config module even when it's defaulting to development, otherwise all implementors would have to wrap the module and provide that code themselves.

[mjlescano]

nodeEnv LGTM