next/previousLabel keep unused li in DOM

[mary139]

1.In README file there was two boolean type, one written Bool, other boolean, meanwhile all other type were written with start Capital letter and complete word .
2.When you don't need to show next/previousLabel, there was only one way : to give mentioned props to null, but this still keeps their Li tag in DOM.

these two are fixed in sync boolean type in README & remove li when don't need show nextLabel #507 pull request.