Support styles in html label

I would like to add this commit for customizing styles in html label as plugin wants.

This aims for brightly showing icons in my package ([autocomplete-emojis](https://github.com/eqot/autocomplete-emojis)) with a customized style which is ```opacity: 1``` instead of ```opacity: 0.3``` by default in autocomplete-plus.

If this does not fit your API style, you can modify this as you like.