Add rtagClass and wtagClass support

[HardlyMirage]

Added two new options rtagClass and wtagClass to allow setting classes on the root and item wrappers. Both options use the same API as Vue's v-bind:class

[tangbc]

@HardlyMirage Thank your pr and fix my poor English on the documentation 😂

I think a prop or attribute named as lowerCase is better than camel-case, for sometimes it will be used in a HTML element instead of .vue template, so I changed the prop name rtagClass to rclass, wtagClass to wclass.