Product output for all files checked

[samskiter]

Feature

An option to have mypy output something for every file checked, regardless of whether any errors are detected.

Pitch

See this comment: #9672 (review)

This would make implementing the VSCode mypy extension a lot easier - right now errors aren't cleared - it would be great if there was an option to have mypy list the files it checked as part of a request.

See this PR/Description for a clear example of where this would be useful: microsoft/vscode-mypy#123 (comment)

Perhaps an output like:

file.py:29:5:39:64: info: file found

Open to any suggestions though