Fix Formatter trait toJson and toVue methods

[stevebauman]

Really like this library so far, thanks for your work on it!

This PR introduces a fix to both the toVue and toJson methods inside the Formatter trait. The toVue method would throw an exception due to json_decode being called on the getSeries() result which returns an array, rather than on the getOptions() result which is a JSON encoded string.

I've also tweaked the config doc to more clearly indicate its purpose and added a link to the docs for user help. In addition, I've also removed some unnecessary @test annotations in the existing PHPUnit tests, as the method names are already prefixed with test.

If we're able to get this merged I'd be happy to make some more PR's adding more documentation and such.

Let me know if you'd like anything changed or adjusted. Thanks for your time! 🙏

[denisdulici]

Thanks Steve for your contribution.