Renamed count to instance_count (fixes #23)

Add missing variable to the usage example

Added aws_eip to example and pre-commit hooks

Add tags to output variables

Fix incorrect subnet_id output expression

This was always producing an error, but prior to Terraform 0.11 these errors were suppressed and shown only in the logs.

Now that Terraform 0.11 shows errors in outputs, they must all be valid in order to use this module.