✨ Support piping in SOURCE's (filepaths and directories)

[CasperWA]

It would be great if one could do something like:

git diff --name-only | entities-service validate

This being thought to work best with the updated input method introduced in #126.

If an option is necessary (like --stdin) this might be a better way of doing it, to ensure the user actively chooses to use the piped in text.