feat: markdown slots (close: #594)

Source:

```md
- Evan You

<template slot="projects">

    - Vue.js
    - VueX

</template>
```

Usage:

```
<Content/> // get default slot
<Content slot="projects"> // get named slot
```

Author: ulivz

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

@@ -13,6 +13,7 @@ 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'
 
@@ -35,6 +36,7 @@ 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

@@ -9,15 +9,19 @@ export default {
       type: Boolean,
       default: true
     },
-    pageKey: String
+    pageKey: String,
+    slot: String
   },
 
   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
+      style: data.style,
+      props: {
+        target: props.slot || 'default'
+      }
     })
   }
 }

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

@@ -0,0 +1,9 @@
+export default {
+  functional: true,
+
+  render (h, { props, slots }) {
+    return h('div', {
+      class: 'content'
+    }, slots()[props.target])
+  }
+}

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

@@ -83,8 +83,9 @@ module.exports = function (src) {
 
   const res = (
     `<template>\n` +
-      `<div class="content">${html}</div>\n` +
+      `<ContentSlotsDistributor :target="target">${html}</ContentSlotsDistributor>\n` +
     `</template>\n` +
+    `<script>export default { props: ['target'] }</script>` +
     (hoistedTags || []).join('\n')
   )
   cache.set(key, res)