Allow project-local configuration, optionally ignoring global config

[clbarnes]

When collaborating on a document or using CI, it's useful for formatters/ fixers to be able to use the same config and no other to get reproducible results across environments. For example, editorconfig allows you to set a local configuration, which has a root field to prevent it looking for config elsewhere.

If the config grows in complexity, merging configurations might get tricky.