parser: Refactor frontmatter parser and add tests

parser: Refactor frontmatter parser and add tests

Lots of cleanups here:

- Refactor InterfaceToConfig and InterfaceToFrontMatter to use io.Writer.
- Simplify InterfaceToFrontMatter by wrapping InterfaceToConfig.
- Export FrontmatterType since we return it in DetectFrontMatter.
- Refactor removeTOMLIdentifier to avoid blindly replacing "+++".
- Update HandleJSONMetaData to return an empty map on nil input.
- Updates vendored goorgeous package and test for org-mode frontmatter.
- Add tests and godoc comments.

Coverage for parser package increased from 45.2% to 85.2%.