Support for NonEscapeCharacter

ES5 allows for non-special characters to be escaped as themselves in strings, however JSON restricts escapes to only special characters.

Compare:

```js
> '\b'
'\b'
> '\c'
'c'
> JSON.parse('"\\b"')
'\b'
> JSON.parse('"\\c"')
SyntaxError: Unexpected token c
```

This patch will allow ES5 [*NonEscapeCharacter*](http://es5.github.io/#x7.8.4) productions in strings in JSON5.

```js
> JSON5.parse('"\\b"')
'\b'
> JSON5.parse('"\\c"')
'c'
```