Added optional 2nd parameter to diffForHumans

When set to true, it modifies the string to only contain the human-readable time difference by omitting modifiers **ago**, **from now**, **before**, and **after**

This is similar to #100, but with passing tests.
In the [contributing](https://github.com/briannesbitt/Carbon#contributing) section it states:

> Change the `readme.src.md` and then use the `readme.php` to generate the new `readme.md` file.

Is that outdated?