Allow existence check of a non *.tf file

[jvanan24]

Hi,

It seems terraform-validator is unable to check if a non *.tf file exists :

 layers:
   default:
     files:
       README.md:
         mandatory: true

 terraform-validator .
INFO: running on terraform/meta with default configuration
ERROR: missing mandatory file(s):
  - README.md

ls -1  terraform/meta/README.md
terraform/meta/README.md

In this sample, it good be interesting to ensure that a README.md file exists