Use .gitignore for filtering

[anderseknert]

Some configuration options to the ignore attribute that would be nice additions:

ignore:
  # both of these would be true by default
  dot-git-dir: true
  use-git-ignore: true
  files:
    - foo.rego

We currently hard code .git and .idea as we never will want to parse whatever is in there, but exposing these options in config, and using the .gitignore file, would be a better approach.

Thanks @srenatus for the idea.