refactor: simplify the implementation of makrdown slots

packages/@vuepress/core/lib/app/app.js

@@ -13,7 +13,6 @@ import('@temp/style.styl')
 
 // built-in components
 import Content from './components/Content'
-import ContentSlotsDistributor from './components/ContentSlotsDistributor'
 import OutboundLink from './components/OutboundLink.vue'
 import ClientOnly from './components/ClientOnly'
 
@@ -36,7 +35,6 @@ Vue.use(Router)
 Vue.mixin(dataMixin(I18n, siteData))
 // component for rendering markdown content and setting title etc.
 Vue.component('Content', Content)
-Vue.component('ContentSlotsDistributor', ContentSlotsDistributor)
 Vue.component('OutboundLink', OutboundLink)
 // component for client-only content
 Vue.component('ClientOnly', ClientOnly)

packages/@vuepress/core/lib/app/components/Content.js

@@ -16,12 +16,11 @@ export default {
   render (h, { parent, props, data }) {
     const pageKey = props.pageKey || parent.$page.key
     Vue.component(pageKey, components[pageKey])
+
     return h(pageKey, {
       class: [props.custom ? 'custom' : '', data.class, data.staticClass],
       style: data.style,
-      props: {
-        target: props.slot || 'default'
-      }
+      slot: props.slot || 'default'
     })
   }
 }

packages/@vuepress/core/lib/app/components/ContentSlotsDistributor.js

-export default {
-  functional: true,
-
-  render (h, { props, slots }) {
-    return h('div', {
-      class: 'content'
-    }, slots()[props.target])
-  }
-}

packages/@vuepress/core/lib/webpack/markdownLoader.js

@@ -83,7 +83,7 @@ module.exports = function (src) {
 
   const res = (
     `<template>\n` +
-      `<ContentSlotsDistributor :target="target">${html}</ContentSlotsDistributor>\n` +
+      `<div class="content">${html}</div>\n` +
     `</template>\n` +
     `<script>export default { props: ['target'] }</script>` +
     (hoistedTags || []).join('\n')