Fixes issue vektra#726.

We needed an additional bit of logic to ensure that if the `config` section
is nil that we default it to being an empty map.

This commit adds a fix where we create a copy of the top-level config map, instead
of using it directly. Previously this situation caused an infinite loop in the
map structure, so creating a copy prevents that from happening.

The test wasn't properly initializing the viper object so the config
wasn't getting decoded into the struct properly.

Also adding more comments to various places to better explain what the
code was doing, as even I, the person who wrote it, was confused as to what
I was doing. How can I expect other people to understand it if I can't?!

fix usage of predeclared identifier

Subpackages were not correctly inheriting their parent package
configuration.