improvement: allow custom config loaders

[afloren-palantir]

Before this PR

Configs were always loaded from hard coded file paths

After this PR

Added a new optional entry point that allows providing custom config loading functions (not necessarily even loaded from files). ==COMMIT_MSG== allow custom config loaders ==COMMIT_MSG==

Possible downsides?

This can't be used with the provided main macro, should we add some sort of new macro or just assume anyone doing this type of configuration knows what they are doing and doesn't need it?

[sfackler]

Needs a rebase but LGTM

[afloren-palantir]

@sfackler should be good to go though it looks like I invalidated the approval by force-pushing the rebase 😬