Include relative files into generated content

Local relative files can be included automatically in the generated
content with `{{ include "..." }} function. This can be very useful
for:

- inject arbitrary markdown files in between sections
- automatically include example usage

````yaml
content: |-
  include any relative files

  {{ include "relative/path/to/file" }}

  or examples

  ```hcl
  {{ include "examples/foo/main.tf" }}
  ```
````

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

examples/.terraform-docs.yml

@@ -24,6 +24,12 @@ sections:
 #
 #   and even in between sections
 #
+#   ## Examples
+#
+#   ```hcl
+#   {{ include "relative/path/to/main.tf" }}
+#   ```
+#
 #   {{ .Providers }}
 #
 #   and they don't even need to be in the default order