Merge plugin config deeply

I'm offering this on Issue #1016, as an alternative to adding the dependency on Hash::Merge::Simple (#1027).
I included a test and documentation updates (including a commit for the original behavior :-). I had already implemented this for a project I am developing; it not only avoids a dependency, but also implement a deep (hash-only) merge, which none of the existing CPAN modules I looked at have.
Note that I patched only the "MERGEABLE" keys' behavior. Not sure if it is necessary or meaningful to
do it for all keys.
You can find a test for the deep_merge algorithm at https://github.com/dmaestro/Util-Hash.git