Add support for grouped count=n resources

[adammck]

This is a little ugly, because the internal data structures weren't really designed for this. I didn't consider resources which use the count attribute to spawn multiple copies. But it works. When run against the updated tfstate fixture:

./terraform-inventory --list fixtures/example.tfstate | jq .

The output is:

{
  "web-aws": [
    "52.7.58.202",
    "52.91.51.56"
  ],
  "web-aws.0": [
    "52.7.58.202"
  ],
  "web-aws.1": [
    "52.91.51.56"
  ],
  "web-cs": [
    "10.2.1.5"
  ],
  "web-cs.0": [
    "10.2.1.5"
  ],
  "web-do": [
    "192.241.136.44"
  ],
  "web-do.0": [
    "192.241.136.44"
  ]
}

Note that resources are now accessible by their exact name, which includes the counter, and as a group by their base name, which does not. Feedback is very welcome.

Hopefully closes #16. It's currently unclear how this relates to #21, so that may well end up merged, too.

[vasiliyb]

👍