indent multiline strings

If a value is a multiline string, the indentation of the output can look jagged. This PR forces the indentation to be used within multiline strings. It also adds a wrapper to make it more clear that this is happening.

Before this PR:

```javascript
var obj = {
  message: "something went wrong:\nit didn't start"
};
console.log(render(ojb), {}, 4);

/*
    message: something went wrong:
it didn't start
*/
```

After this PR:

```javascript
var obj = {
  message: "something went wrong:\nit didn't start"
};
console.log(render(ojb), {}, 4);

/*
    message: 
      """
        something went wrong:
        it didn't start
      """
*/
```

I'm open to thoughts on the exact formatting. I could really use this feature in general, though.