Added support for passing class names to portal-target component.

[forejerk]

Hi. Thank you for this great project :-).
This portal-vue project has brought happiness into my life and helped me lose weight.
#101 (Class passed to component is lost in the portal)
I have added some code to solve this issue. I I decided to write it as an issue because not be useful or it might be wrong.
It code is working well on my private project.

src/components/portal.tsx

    sendUpdate() {
      const slotContent = this.normalizeSlots()
      
      // after... inserted.addClass
      this.$nextTick(()=>{
        if (slotContent) {
          const transport: TransportInput = {
            from: this.name,
            to: this.to,
            /*
             * Example
             *
             *   <portal to='example' class='sample-class' v-addclass.sample>
             *  </portal>
             *  
             *    1.this.$vnode.data.staticClass
             *    2.this.$vnode.context.$el.className
             *  
             *   Vue.directive('addclass',{
             *     inserted:function(el,binding,vnode){
             *       $(el).addClass(vnode.context.$options.name)
             *       $(el).addClass(Object.keys(binding.modifiers))
             *     }
             *   })
             * 
             */
            classes: [this.$vnode.context ? this.$vnode.context.$el.className : '',this.$vnode.data ? this.$vnode.data.staticClass : ''].filter(Boolean).join(' ') , // add
            passengers: [...slotContent],
            order: this.order,
          }
          wormhole.open(transport)
        } else {
          this.clear()
        }
      })
    }

src/components/wormhole.ts

export const Wormhole = Vue.extend({
  data: () => ({
    transports,
    targets,
    sources,
    trackInstances: inBrowser,
  }),
  methods: {
    open(transport: TransportInput) {
      if (!inBrowser) return
      const { to, from, classes, passengers, order = Infinity } = transport // add
      if (!to || !from || !passengers) return

      const newTransport = {
        to,
        from,
        classes, // add
        passengers: freeze<object>(passengers),
        order,
      } as Transport

src/components/portal-target.tsx

  computed: {
    ownTransports(): Transport[] {
      const transports: Transport[] = this.transports[this.name] || []
      if (this.multiple) {
        return transports
      }
      return transports.length === 0 ? [] : [transports[transports.length - 1]]
    },
    classes():String[] { // add
      /*
       *
       * Example
       *  
       *  When portaling multiple elements' from.elems -> to.target { multiple:true } 
       * 
       *  from
       *    <portal name="destination" :order="2" class='two'> <p>some content</p> </portal>
       *    <portal name="destination" :order="1" class='one> <p>some other content</p> </portal>
       *  to
       *    <portal-target name="destination" multiple="true">
       * 
       *  Mount with all class names assigned.
       *  
       *    <div class="one two vue-portal-target">
       *      <p>some other content</p>
       *      <p>some content</p>
       *   </div>
       * 
       */
      return this.ownTransports.map(o=>o.classes)
    },

src/types.ts

export interface TransportInput {
  to: string
  from: string
  classes?: string, // add
  order?: number
  passengers: Array<VNode | Function>
}