dayamlchecker-fmt could normalize case of all top-level keys to lowercase

[nonprofittechy]

See #23 for more detailed discussion. Docassemble allows keys like Question, Fields etc, but they aren't standard.

Our formatter could normalize those to lower case.

Maybe should discuss any possible downsides before implementing.

(certainly will need some heuristic to decide if a YAML is meant to be a Docassemble YAML or not)