Some smallish updates

Hi.

I'm not sure if you want updates in the official repo or if you'd rather people run with it. But the changes I made are really quite generic and you might like them. It's all in the commits ofc but here is the short list:

* Changed formatters to use `call` instead of `format` so one can use procs and lambdas as formatters.
* Removed use of global `$;` in favor of a `Differ.separator` class constant.
* Updated Readme to reflect that you can use regex as separator (including the difference between including a capture group and not).
* Removed rspec config block (since it was empty and used the deprecated syntax).
* Revamped the readme and converted it to markdown.

If you like it please merge. I will add some new features in the coming weeks and would be happy to restructure the code a bit more, if you want.

All the best!