Add description and table entries for FormatJS

I added the description and updated the table for [FormatJS](http://formatjs.io/).

Closes #6
Given format.js's addresses each of this functionalities using a different library, what about listing them? Date formatting is supported by ECMA-402, right? Suggestion:

```diff
- :heavy_check_mark:
+ :heavy_check_mark:<br><sub><sup>Via&nbsp;ECMA-402 or its polyfil</sup></sub>
```

PS: If this is too work, let me know the libraries and I can change this later.