Better support for templates with frontmatter

This adds support for excluding frontmatter settings common in many static site generators like Middleman and Jekyll. I didn't see equivalent of RuboCop's AllCops config setting, so I added a global option. I'm open to renaming it AllLinters if we want to follow RC's lead. The frontmatter support is controlled by a new global config option 'skip-frontmatter', which defaults to false so it's opt-in and shouldn't break anyone.