Add format function from the README

[HoldYourWaffle]

I based this PR of #3 so I could immediately update the type definitions

I added the formatting function from the README to the library with some additional options for black/whitelisting certain properties and changing the 'glue' used.
Since this is a breaking change (different imports) I also updated the major version number for your convenience.

If you decide to merge this I could also update the documentation (and include a short 'migration' section).

[nikitaeverywhere]

Hello! Thank you so much for your work, I appreciate that!

I've reviewed your pull requests #3 & #4, they're awesome, thanks! I'll merge them in a short while.

However, this PR looks a bit strange to me. The point of those lines of code I put in the readme regarding formatting was just to mention the quick example of how to use the library. The formatting is not something that this library deserves, I have a strong opinion about that. The formatting is a thing where people usually end up using a whole another library (for example), for a reason: you cannot write a function which covers a will of all the library users. Check how much readable it turns to be while using the library:

import getDateDiff from "datetime-difference";
import format from "string-format";

const dateDiff = getDateDiff(new Date(), new Date()); // ...
console.log(
    format("{days} days left", dateDiff)
);

No glue, no custom functions, the freedom in choosing how to format a thing. (this PR also breaks the compatibility with node v4)

So I would strongly advise you to follow this path, keeping the library as minimalistic as possible. What makes sense though, is to make a PR adding the example above to the readme file :) Done

Thanks!