Add rule generator

[wata727]

Add a rule generator to help new contributors add rules. This is a modified version of the script that previously existed in the core repository. you can use a generator like the following:

$ go run ./rules/generator
Rule name? (e.g. aws_instance_invalid_type): aws_instance_example_type
Created: rules/aws_instance_example_type.go
Created: rules/aws_instance_example_type_test.go
Created: docs/rules/aws_instance_example_type.md
Modified: rules/provider.go

TODO:
1. Remove all "TODO" comments from generated files.
2. Write implementation of the rule.
3. Add a link to the generated documentation into docs/rules/README.md

[chaspy]

Awesome!

I've run the generator and read the TODO comments. Contributor friendly 👍