[CONFIGURATION-524] Added support for cloning a ConfigurationInterpolator.

If a configuration is cloned, its interpolator has to be cloned, too, because
it typically contains a Lookup pointing to the original configuration.
AbstractConfiguration now provides default functionality for this purpose
which can be called by clone() implementations in derived classes.
BaseConfiguration already calls this method.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1467445 13f79535-47bb-0310-9956-ffa450edef68