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%.

commands/new.go

@@ -356,15 +356,11 @@ func createConfig(fs *hugofs.Fs, inpath string, kind string) (err error) {
 	}
 	kind = parser.FormatSanitize(kind)
 
-	by, err := parser.InterfaceToConfig(in, parser.FormatToLeadRune(kind))
+	var buf bytes.Buffer
+	err = parser.InterfaceToConfig(in, parser.FormatToLeadRune(kind), &buf)
 	if err != nil {
 		return err
 	}
 
-	err = helpers.WriteToDisk(filepath.Join(inpath, "config."+kind), bytes.NewReader(by), fs.Source)
-	if err != nil {
-		return
-	}
-
-	return nil
+	return helpers.WriteToDisk(filepath.Join(inpath, "config."+kind), &buf, fs.Source)
 }