Make the title case style guide configurable

This works for the `title` func and the other places where Hugo makes title case.

* AP style (new default)
* Chicago style
* Go style (what we have today)

Fixes #989

helpers/general.go

@@ -26,6 +26,8 @@ import (
 	"unicode"
 	"unicode/utf8"
 
+	"github.com/jdkato/prose/transform"
+
 	bp "github.com/gohugoio/hugo/bufferpool"
 	"github.com/spf13/cast"
 	jww "github.com/spf13/jwalterweatherman"
@@ -194,6 +196,29 @@ func ReaderContains(r io.Reader, subslice []byte) bool {
 	return false
 }
 
+// GetTitleFunc returns a func that can be used to transform a string to
+// title case.
+//
+// The supported styles are
+//
+// - "Go" (strings.Title)
+// - "AP" (see https://www.apstylebook.com/)
+// - "Chicago" (see http://www.chicagomanualofstyle.org/home.html)
+//
+// If an unknown or empty style is provided, AP style is what you get.
+func GetTitleFunc(style string) func(s string) string {
+	switch strings.ToLower(style) {
+	case "go":
+		return strings.Title
+	case "chicago":
+		tc := transform.NewTitleConverter(transform.ChicagoStyle)
+		return tc.Title
+	default:
+		tc := transform.NewTitleConverter(transform.APStyle)
+		return tc.Title
+	}
+}
+
 // HasStringsPrefix tests whether the string slice s begins with prefix slice s.
 func HasStringsPrefix(s, prefix []string) bool {
 	return len(s) >= len(prefix) && compareStringSlices(s[0:len(prefix)], prefix)