Add support for VPN connection routes

[ctiwald]

More resources for #28. I believe this also completely closes out #1011.

The acceptance tests pass locally and docs are included with this PR. I thought briefly about making this a configuration option on the vpn_connection resource but resisted for a few reasons. First, AWS's API makes these distinct calls, and while it's quite frustrating as an AWS API consumer, users of the AWS provider will probably look to CloudFormation or the API docs to determine what resources they need to build when picking up terraform. Additionally, these are static routes over a VPN tunnel, which means that they need to be deliberately added and removed to avoid network interruptions. If we located their update logic inside resource_aws_vpn_connection.go, I believe we'd have to delete all rules, then reapply the newly discovered ones, much the way security groups handle ingress/egress. That's really not ideal for this type of resource, hence, I made it its own thing.

We can try to protect users from AWS's kludgey API, but only so far. Making this its own resource requires a bit more configuration by the user but will result in a more reliable resource.

[mitchellh]

Perfect. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.