code: func interpolationFuncTitle() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			toTitle := args[0].(string)
  			return strings.Title(toTitle), nil
  		},
  	}
  }
docs: // interpolationFuncTitle implements the "title" function that returns a copy of the
  // string in which first characters of all the words are capitalized.