feat: `tpl` template function (#504)

Add `tpl` template function that looks exactly like helm's one.

Resolves #420

tmpl/context_funcs.go

@@ -24,6 +24,7 @@ func (c *Context) createFuncMap() template.FuncMap {
 		"requiredEnv":    RequiredEnv,
 		"get":            get,
 		"getOrNil":       getOrNil,
+		"tpl":            c.Tpl,
 	}
 	if c.preRender {
 		// disable potential side-effect template calls
@@ -128,6 +129,14 @@ func (c *Context) ReadFile(filename string) (string, error) {
 	return string(bytes), nil
 }
 
+func (c *Context) Tpl(text string, data interface{}) (string, error) {
+	buf, err := c.RenderTemplateToBuffer(text, data)
+	if err != nil {
+		return "", err
+	}
+	return buf.String(), nil
+}
+
 func ToYaml(v interface{}) (string, error) {
 	data, err := yaml.Marshal(v)
 	if err != nil {

tmpl/context_funcs_test.go

@@ -118,3 +118,18 @@ func TestSetValueAtPath_TwoComponents(t *testing.T) {
 		t.Errorf("unexpected result: expected=%v, actual=%v", expected, actual)
 	}
 }
+
+func TestTpl(t *testing.T) {
+	text := `foo: {{ .foo }}
+`
+	expected := `foo: FOO
+`
+	ctx := &Context{basePath: "."}
+	actual, err := ctx.Tpl(text, map[string]interface{}{"foo": "FOO"})
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("unexpected result: expected=%v, actual=%v", expected, actual)
+	}
+}