Add line output [experimental]

Note: relatively hacky, sketchy, weird code since I'm a parsing noob.

This adds non-enumerable `__line__` properties to objects and arrays so that higher-level validators can give more accurate tracebacks.

It can't do so with numbers and booleans because javascript.

Maybe this would make sense as an option in jsonlint, if it could be made an option.

Example:

```js
var lint = require('./');

var obj = lint.parse('{"foo":2,\n\n"bar":{},"ex":"no",\n"list":[]}');
console.log('obj', obj.__line__);
console.log('bar', obj.bar.__line__);
console.log('list', obj.list.__line__);

/*
obj 1
bar 3
list 4
*/
```