ignore global constructs like Math, Object, Array, JSON, & Date

Before:

```js
props('Math.round(n) + JSON.stringify(blob)', '$.') 
// $.Math.round($.n) + $.JSON.stringify($.blob)
```

Now:

```js
props('Math.round(n) + JSON.stringify(blob)', '$.') 
// Math.round($.n) + JSON.stringify($.blob)
```

--- 

I didn't add an exhaustive list, but this should get it started. A good list is at: [MDN: Javascript Reference](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference). I can add them all, but I figured these were the most common ones that would come through.