* add cidr_block outputs to public and private subnets

* add cidr_block outputs to database and elasticache subnets

* add redshift subnet option

* fix tag

* add missing route association for redshift

* add vpc_endpoint_s3_pl_id and vpc_endpoint_dynamodb_pl_id outputs

* add redshift to complete-vcp example

* fix domain_name_servers - keep it a list

* fix outputs for TF 0.11

* fix missing *

* Minor corrections in example and in type of dhcp_options_domain_name_servers

* Minor corrections in example

* Updated README

[CodeTriage](https://www.codetriage.com/) is an app I have maintained
for the past 4-5 years with the goal of getting people involved in
Open Source projects like this one. The app sends subscribers a random
open issue for them to help "triage". For some languages you can also
suggested areas to add documentation.

The initial approach was inspired by seeing the work of the small
core team spending countless hours asking "what version was
this in" and "can you give us an example app". The idea is to
outsource these small interactions to a huge team of volunteers
and let the core team focus on their work.

I want to add a badge to the README of this project. The idea is to
provide an easy link for people to get started contributing to this
project. A badge indicates the number of people currently subscribed
to help the repo. The color is based off of open issues in the project.

Here are some examples of other projects that have a badge in their
README:

- https://github.com/crystal-lang/crystal
- https://github.com/rails/rails
- https://github.com/codetriage/codetriage

Thanks for building open source software, I would love to help you find some helpers.

* Added fix for issue when no private subnets are defined

* Minor readme

* Allow passing in EIPs for the NAT Gateways

* Document how to pass external IPs for NAT Gateways

* Update README.md to reduce example code

Outputs directly referencing resource IDs that are only conditionally
created are not valid. Use the concat * trick to make sure we always
have a valid value for these outputs.

* Add support for DHCP options set

* code cleanup

* remove unnecessary depends_on in aws_vpc_dhcp_options_association definition

Add AZ to nat gateway name

NAT gateway should be tagged too.

Improve variable descriptions

disable dynamodb data source when not needed

Allow the user to define custom tags for route tables