* 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