Add terraform_required_providers rule

[bendrucker]

This PR adds a rule that checks for terraform.required_providers and falls back to provider.version if set. It borrows copy from the Terraform docs:

https://www.terraform.io/docs/configuration/providers.html#provider-versions

[mveitas]

👍 I recently added a rule to enforce that the terraform required_version attribute be set for a module and thought about making it a rule to perform enforcement of the version and the providers within the terraform block. Separating this out keep the code simple and easy to understand!

[mveitas]

You are going to need to add the rule here: https://github.com/mveitas/tflint/blob/master/rules/provider.go

[bendrucker]

Thanks, will do!

[bendrucker]

Fixed the two mistakes in the docs, thanks!

[wata727]

👍