Don't re-check style on unchanged code

[jackwasey]

Currently, it seems that every file, is checked regardless of whether it is updated. Would it be reasonable to check file timestamps and cache them either in the package or in a tidy place in the user's home directory, or even $TMP? Or use digest to cache the hashes instead of timestamps? This would save a lot of time for bigger R packages, and make this easier to run styler frequently, or possibly automatically with each git commit (if only to check whether styling is due). Q.v. #467

[lorenzwalthert]

I think this is a duplicate of #320, which obviously is hard to find. Do you mean the environment variable $TMP? I think that's not a good idea because it is not persistent. In #320, we had digest in mind as well. However, I am not sure what the best place to store this information is. Let's continue in #320.