Raise on override defined config

[jobertabma]

All PRs:

• Has tests
• Documentation updated

This PR changes the behavior of the override and named_append methods to raise an exception when it is called more than once with the same configuration name. This is to avoid unexpected behavior in case a configuration is used in some way in between two configuration declarations.

Fixes #435

[oreoshake]

Open question: should the named_append method also check the existence of a configuration in the @overrides hash and vice versa?

🤔 it'd probably be confusing and I can't think of a reason why you'd want overrides and appends to have the same name, or at least I think you'd be OK with making the required change once your app stopped booting?

[jobertabma]

@oreoshake I've updated the PR based on your comment, should be ready for review. Thanks!

[oreoshake]

LGTM thanks!