Add plainify template function

To strip away any HTML. May be useful for the .Title in head etc.

People may shoot themself in the foot with this, maybe ...

The replacement function is pretty fast.

tpl/template_funcs.go

@@ -1169,9 +1169,21 @@ func emojify(in interface{}) (template.HTML, error) {
 	if err != nil {
 		return "", err
 	}
+
 	return template.HTML(helpers.Emojify([]byte(str))), nil
 }
 
+// plainify strips any HTML and returns the plain text version.
+func plainify(in interface{}) (string, error) {
+	s, err := cast.ToStringE(in)
+
+	if err != nil {
+		return "", err
+	}
+
+	return helpers.StripHTML(s), nil
+}
+
 func refPage(page interface{}, ref, methodName string) template.HTML {
 	value := reflect.ValueOf(page)
 
@@ -1674,6 +1686,7 @@ func init() {
 		"mul":          func(a, b interface{}) (interface{}, error) { return helpers.DoArithmetic(a, b, '*') },
 		"ne":           ne,
 		"partial":      partial,
+		"plainify":     plainify,
 		"pluralize":    pluralize,
 		"readDir":      readDir,
 		"ref":          ref,

tpl/template_funcs_test.go

@@ -108,6 +108,7 @@ safeHTML: {{ "Bat&Man" | safeHTML | safeHTML }}
 safeCSS: {{ "Bat&Man" | safeCSS | safeCSS }}
 safeURL: {{ "http://gohugo.io" | safeURL | safeURL }}
 safeJS: {{ "(1*2)" | safeJS | safeJS }}
+plainify: {{ plainify  "Hello <strong>world</strong>, gophers!" }}
 `
 	expected := `chomp: <p>Blockhead</p>
 dateFormat: Wednesday, Jan 21, 2015
@@ -151,6 +152,7 @@ safeHTML: Bat&Man
 safeCSS: Bat&amp;Man
 safeURL: http://gohugo.io
 safeJS: (1*2)
+plainify: Hello world, gophers!
 `
 
 	var b bytes.Buffer