Add a new flag --out for saving a plan file after dry-run migrations

[minamijoyo]

Fixes #36

Add a new flag --out for saving a plan file after dry-run migrations.
It almost equivalents to terraform plan -out, but runs after dry-run migrations. Note that the new flag is --out, not -out. This is only because the pflag library we use follows the GNU style long options.

This is intuitively simple, but after implemented, it turned out it wasn't a trivial matter. The plan file internally contains terraform configurations used by plan. The tfmigrate uses a temporary local backend configuration not to change a remote state in plan phase. This means applying the plan file affects only a local state.

We need the following steps to apply the plan:

Given a migration file with force attribute.
(1) tfmigrate plan --out=foo.tfplan tfmigrate_test.hcl
(2) tfmigrate apply tfmigrate_test.hcl
(3) terraform apply foo.tfplan
(4) terraform state push -force terraform.tfstate
(5) terraform plan -detailed-exitcode
(6) rm terraform.tfstate foo.tfplan

Make sure to force push the local state to remote after terraform apply. The -force flag is required in (4) because the lineage of the state will be changed. As you know, this is unsafe operation. But I couldn't find a safe way to do this in the current technical limitations. You can confirm the final plan has no changes in (5).

There are some limitations:

1. terraform show foo.tfplan rejects it as a stale plan. Option to not error when showing a stale Terraform plan hashicorp/terraform#28136
2. terraform state push -force terraform.tfstate doesn't work in Terraform >= v0.12.25 and < v0.13 Terraform remote state push -force fails lineage check for S3 backend hashicorp/terraform#25761