Add option to give custom highlight class to item as prop

[vivekitis]

• vue-autosuggest version: 2.1.1
• node version:
• npm (or yarn) version:

Relevant code or config

                [`${componentAttrPrefix}__results-item--highlighted`]: isHighlighted

What you did:

What happened:

Reproduction sandbox/repository:

Problem description:
Feature Request.
Right now Highlight class can only be changed overriding the className in css that is added by the component. But there needs to be a provision to pass className as a prop to attach to elem at currentIndex, because there are cases when the exact classname is only known after build like using module level scope in vue styles.

Suggested solution:
Added Pr. #188.
please suggest any improvements if needed. can move prop to sectionConfig if needed.