Add "Lint" mode

[KostyaEsmukov]

Currently scaraplate rollup doesn't have a no-action mode, i.e. it always writes the changes. It might be useful to add a no-action mode, which just checks if there would be any changes made by rollup and prints diff and sets an exit code (0 -- success -- for no changes; and 1 -- error -- for any changes).

This mode can be used in CI pipelines to ensure that new changes didn't introduce any new diff from a template.

As for design, I like the one used by black. There're 2 flags:

  --check                         Don't write the files back, just return the
                                  status.  Return code 0 means nothing would
                                  change.  Return code 1 means some files
                                  would be reformatted.  Return code 123 means
                                  there was an internal error.
  --diff                          Don't write the files back, just output a
                                  diff for each file on stdout.