Implement -chdir option to mimic terraform behaviour

[sergei-ivanov]

tflint --help says:

Usage:
  tflint [OPTIONS] [FILE or DIR...]

...and I assumed that if I pass a directory to it, that directory will be considered root for scanning. But apparently it's not the case, the root is still the current directory, which means I always need to cd into the target directory before running tflint.

I guess we need a separate option like -chdir in Terraform, as was also mentioned here. It would also allow scripting recursive checks without the need for cd or pushd/popd for every module.

[bendrucker]

Of note, Terraform handles this option different than all the rest of its command options. While command options follow the command (e.g., terraform apply -target), -chdir modifies all commands and comes first (terraform -chdir=./foo apply). IIRC Terraform does some custom flag parsing to achieve this and we'd probably borrow that code.

[bendrucker]

See #1580 for an instance of this difference in behavior tripping up users. In Terraform, file("./foo") works when you call terraform -chdir=/my/module apply. In TFLint, where the working directory is not actually changed, this call fails.