Feature Request: Cached outputs

[ultrabear]

Preface: I have a large codebase that I run yapf over, it takes a long time to run over all the code so I thought about methods of speeding it up

This is a feature request to yapf to have a system similar to pytype/mypy where it caches the same file under the same settings so it does not need to re-scan them if they are unchanged, speeding up yapf on large codebases