Add ec2_vpc_igw module for independently enabling/disabling VPC Internet Gateways.

This is a new module which ensures presence/absence of an Internet Gateway for an EC2 Virtual Private Cloud.

This module supports `check_mode` and supports returning the IGW's `gateway_id`.

This is motivated by some work I've done that will be in a future pull request which starts to split out the monolithic `ec2_vpc` module in order to increase idempotency and flexibility of VPC features such as subnets, routing tables, and security groups.