Use lodash for deep cloning.

This way Date, Regexp and other non-plain objects
are preserved while copying.

Fixes #139.
The [util.cloneDeep()](https://github.com/lorenwest/node-config/wiki/Using-Config-Utilities#clonedeepcopyfrom-depth) API cannot change without introducing a backward incompatibility.

According to the [Future Compatibility](https://github.com/lorenwest/node-config/wiki/Future-Compatibility) spec, a 1 year waiting period would be required for backward incompatible changes.