"Conservative" option to disable formatting for existing, non-compliant files

[clbarnes]

Sometimes maintainers get upset if you trash their diffs on a large file because you've made a small change and then your editor has reformatted the rest of the file. I want my new files to be blackened, and existing compliant files to be kept that way as I make changes, but I don't want to implicitly reformat a file which failed black --check before I touched it.