Detect YAML front matter on any file, regardless of if it is .md or within `_posts`.

This allows proper metadata detection of YAML front matter on jekyll pages, as well as non-markdown files such as having YAML front matter on an HTML template.

Adds function _hasMetadata to utils, and uses the same YAML frontmater regex to detect'
Merge pull request #269 from benbalter/frontmatter-all-the-things

Detect YAML frontmatter on any file, regardless of type