Remove occurrences in repl
Make it clearer that we're copying from config to config
Move to a fall-through config hash.

This is somewhat easier to understand than the current solution,
but does have some downsides: namely it doesn't work for things like
Pry::Hooks.

It is however the only way I've thought of to let setting
Pry.config.color = work in addition to _pry.config.color =.

For now I've added a hack so that hooks work as they did before,
in future we should work out how to fix them so that only the data
for hooks is stored in the config object, not the hooks object itself.
No more Pry.config in documentation helpers