* Allow specifying a list of subnet IDs to iterate instances over

* Include the module version and some code formatting

* Explicitly specify the version for SG AWS module

Any change in the ebs block device currently triggers re-creation of
this resource which I assume is not intended.

Workaround by ignoring these changes as suggested in:

https://github.com/terraform-providers/terraform-provider-aws/issues/1297#issuecomment-319359965

This narrows the match for things considered to be t2 instances to only
those types that start with "t2.". Previously there was a class of
(invalid) instance types, such as "2b.something" or "t5.something" that
would cause an error on boolean parsing by matching one regex and not
the other.

* Added support for CPU credits

* Updated formatting

Modify tag name management to add -%d suffixe only if instance_count > 1

Stop ignoring changes in vpc_security_group_ids

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.