Make terraform.Module available in content

Add one extra special variable the `content`:

- `{{ .Module }}`

As opposed to the other variables, which are generated sections based on
a selected formatter, the `{{ .Module }}` variable is just a `struct`
representing a Terraform module.

It can be used to build highly complex and highly customized content:

```yaml
content: |-
  ## Resources

  {{ range .Module.Resources }}
  - {{ .GetMode }}.{{ .Spec }} ({{ .Position.Filename }}#{{ .Position.Line }})
  {{- end }}
```

Signed-off-by: Khosrow Moossavi <khos2ow@gmail.com>

examples/.terraform-docs.yml

@@ -32,6 +32,11 @@ sections:
 #
 #   and even in between sections
 #
+#   ## Resources
+#   {{ range .Module.Resources }}
+#   - {{ .GetMode }}.{{ .Spec }} ({{ .Position.Filename }}#{{ .Position.Line }})
+#   {{- end }}
+#
 #   ## Examples
 #
 #   ```hcl