Allow limiting the scan to selected directories. If no directory
is provided, the current directory will be scanned, as it was.

BREAKING CHANGE: If you passed a single directory to denolint to
specify the project root, it will not work any more. You can pass
the root directory using the argument -r,--root. However, it was
used only if neither .denolintignore nor .eslintignore were found
and was passed instead of either of them to the walker. Because
the root directory was no file path, I think that it was useless.
Nevertheless, I implemented the previous functionality using the
argument -r,--root, which might be worth for other purposes later.