code: func interpolationFuncIndent() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeInt, ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			spaces := args[0].(int)
  			data := args[1].(string)
  			pad := strings.Repeat(" ", spaces)
  			return strings.Replace(data, "\n", "\n"+pad, -1), nil
  		},
  	}
  }
docs: // interpolationFuncIndent indents a multi-line string with the
  // specified number of spaces