Initial implementation of Strict mode.

* Off by default. Set process.env.NODE_CONFIG_STRICT_MODE to a true value
to enable.
  * When on, there must be an explicit config file matching NODE_ENV if
NODE_ENV is set.
  * When on, there must be an explicit config file matching NODE_APP_INSTANCE
if NODE_APP_INSTANCE is set
  * When on, NODE_ENV must not match 'default' or 'local' to avoid ambiguity.

If a strictness check fails, an exception will be throws at require-time.

If this is excepted, I would update the wiki notes to note that the feature is
experimental and the strictness rules are subject to change.