code: func interpolationFuncBase64Encode() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			s := args[0].(string)
  			return base64.StdEncoding.EncodeToString([]byte(s)), nil
  		},
  	}
  }
docs: // interpolationFuncBase64Encode implements the "base64encode" function that
  // allows Base64 encoding.