feat($core): safer class name of content outlet

We shouldn't let the core render the `.content` class, so once we use custom layout in the default theme, the styles of default theme will pollute the styles of custom layout.

Author: ulivz

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

@@ -8,8 +8,7 @@ export default {
     return h('div',
       {
         class: [
-          'content',
-          props.slotKey
+          `content__${props.slotKey}`
         ]
       },
       slots()[props.slotKey]

packages/@vuepress/theme-default/components/Home.vue

@@ -38,7 +38,7 @@
       </div>
     </div>
 
-    <Content class="custom"/>
+    <Content class="content custom"/>
 
     <div
       class="footer"

packages/@vuepress/theme-default/components/Page.vue

@@ -2,7 +2,7 @@
   <main class="page">
     <slot name="top"/>
 
-    <Content/>
+    <Content class="content"/>
 
     <footer class="page-edit">
       <div