AWS Tag Support

This PR adds tag support to AWS. Specifically, this PR adds it to `aws_vpc`, but shows the basic generic framework I've created to make adding this easier to other AWS resources. After this PR is merged, I'll work directly on master to add it to other resource types.

It looks like this:

```
resource "aws_vpc" "foo" {
    cidr_block = "10.0.0.0/16"

    tags {
        Name = "a name"
    }
}
```