EIP module refactoring (cloudposse#81)

* gateway and instance now use same EIP module
* formatting code
* readme rebuild

Fix EIP NAT instances (cloudposse#80)

Added EIP for NAT instnaces (cloudposse#78)

* Added EIP for NAT instnaces

* Added EIP for NAT instnaces

* Added EIP for NAT instnaces

Fix EIP NAT instances (cloudposse#79)

* Fix EIP NAT instances

* Fix EIP NAT instances

* Remove state because it could be from other version

* Remove state because it could be from other version

* Remove state because it could be from other version

* Remove state because it could be from other version

* Remove state because it could be from other version

Added EIP for NAT instnaces (cloudposse#77)

* Added EIP for NAT instnaces

* Fix README

fix: Remove quotes from references (cloudposse#76)

Terraform 0.12.14+ started warning about deprecated usage (quotes on
references)

Remove unused "region" variable from docs (cloudposse#74)

* Remove unused "region" variable from docs

Historically "region" may have been a variable that could be passed to the `terraform-aws-dynamic-subnets` module, but currently it looks like it no longer is. This PR removes usage of the variable from documentation.

* PR: Rebuild README using Make

Properly pin Terraform providers (cloudposse#68)

Use `required_providers` to pin Terraform providers (cloudposse#67)

Add `bats` and `terratest` for the example (cloudposse#66)