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%.

hugolib/page.go

@@ -1424,15 +1424,20 @@ func (p *Page) SetSourceMetaData(in interface{}, mark rune) (err error) {
 		}
 	}()
 
-	var by []byte
+	buf := bp.GetBuffer()
+	defer bp.PutBuffer(buf)
 
-	by, err = parser.InterfaceToFrontMatter(in, mark)
+	err = parser.InterfaceToFrontMatter(in, mark, buf)
 	if err != nil {
 		return
 	}
-	by = append(by, '\n')
 
-	p.Source.Frontmatter = by
+	_, err = buf.WriteRune('\n')
+	if err != nil {
+		return
+	}
+
+	p.Source.Frontmatter = buf.Bytes()
 
 	return
 }