Add support for specifying --chdir multiple times

[amartani]

Add support for specifying --chdir multiple times. This reuses the current support for running multiple subprocesses in parallel, used by --recursive. With this option, we can now run multiple --chdir without running all --chdir, which is useful, for example, in pre-commit or CI systems on PRs where you may want to run on all modified roots rather than all roots.

[wata727]

Since the --chdir is not originally intended to be used in multiple times, integrity with other features is important. For example, what happens if you use the --filter or --recursive with multiple --chdir commands?

Furthermore, the validity of allowing multiple --chdir needs to be considered. While --chdir follows the design of Terraform, Terraform only allows one --chdir. Unlike Terraform, is there a design justification for allowing multiple --chdir?

[amartani]

The justification for multiple --chdir is the same as for --recursive: you generally want to run tflint on many roots (in CI for example), while when running raw terraform, you want to be more specific. This implementation is just adding more flexibility to that - instead of every root with --recursive, you may want to run only on a subset of roots.

Multiple --chdir with --filter works in the same way as --recursive with --filter - each worker independently resolves the filter on their own directory.

Multiple --chdir with --recursive will run on the union of all the directories under all --chdirs.