Ability to tag route tables

I've tested this functionality in a local integration test, in which I checked no tagging vs. these two different formats:

```
# format 1
resource_tags:
  Environment: Dev

# format 2
resource_tags: { "Environment":"Dev" }
```

One thing that isn't addressed is removing tags, but the other resources (e.g. subnets) don't expose that functionality yet either. Will tackle that in a separate, future PR.