feat(plan): add the option -ignore-warning

[suzuki-shunsuke]

Add the option -ignore-warning to the tfcmt plan command.
You can also configure this option in your configuration file.

terraform:
  plan:
    ignore_warning: true

By default, tfcmt emphasizes the warning of terraform plan, and the option -skip-no-changes is ignored if terraform plan outputs warning.

This is because we think warning should not be ignored.

But actually we're aware that some users ignore warning normally and they feel warning is noisy.
In this case, it's desirable that tfcmt plan -skip-no-changes doesn't post a comment even if warning is outputted.

So this pull request introduces a new option -ignore-warning.

If terraform plan outputs warning, this option makes the template variable .Warning empty, removing the warning from the default template.

If terraform plan -skip-no-changes has no change and outputs warning, this option disables to post a comment.