Update TFLint to v0.12.1

Add support for `ipv6_cidr_block` in `*_{in|out}bound_acl_rules`.  As a
conseqeunce, the (ipv4) `cidr_block` is made optional.

Add VPC endpoints for CloudFormation, CodePipeline, Storage Gateway, AppMesh, Transfer, Service Catalog & SageMaker(Runtime & API) (#324)

* add new endpoints

* merge change from master

* Added icmp_code and icmp_type values to non default acl rules.

* Added support for both ICMP and non-ICMP rules in NACL

* Add tags to VPC Endpoints

* Update variables.tf

add new line between variables

* centralize vpce tag param

* fix s3 tags

* Updated README

* Updated README

* Updated README

* AWS Glue VPC endpoint

* sagemaker notebook vpc endpoint

* correct service name for notebook endpoint

* added outputs kinesis

* added endpoints output glue, fix readme

* endpoint for STS

* endpoint for STS (variables)

* Added more VPC endpoints (Glue, STS, Sagemaker Notebook), and all missing outputs

* IPv6 support

Add variable "enable_ipv6" to allow enabling IPv6 support (resulting in
passing "assign_generated_ipv6_cidr_block" to aws_vpc.

Enabling IPv6 support further results in an Egress-only internet gateway
being provisioned and routing tables of subnets being adjusted.

Additional variables allow to choose the indices out of the /64 subnets
based on the assigned /56 range.

* Add example for IPv6 usage

* Remove redundant parameter assign_generated_ipv6_cidr_block

This is needed exactly when var.enable_ipv6 is true.

* Set subnet ipv6_cidr_block to null if unused

* Be picky about spelling

* Revert unrelated change

* More IPv6 spelling

* Added IPv6 support to VPC module

* Added IPv6 support to VPC module