feat: support excerpt extraction with <!-- more --> (close #174)

Author: yyx990803

docs/guide/custom-themes.md

@@ -54,6 +54,10 @@ If the user provided `themeConfig` in `.vuepress/config.js`, it will also be ava
 
 Finally, don't forget that `this.$route` and `this.$router` are also available as part of Vue Router's API.
 
+## Content Excerpt
+
+If a markdown file contains a `<!-- more -->` comment, any content above the comment will be extracted and exposed as `$page.excerpt`. If you are building custom theme for blogging, this data can be used to render a post list with excerpts.
+
 ## Content Outlet
 
 The compiled content of the current `.md` file being rendered will be available as a special `<Content/>` global component. You will need to render it somewhere in your layout in order to display the content of the page. The simplest theme can be just a single `Layout.vue` component with the following content:

docs/zh/guide/custom-themes.md

@@ -54,6 +54,10 @@ VuePress 使用单文件组件来构建自定义主题。想要开发一个自
 
 最后，别忘了，作为 Vue Router API 的一部分，`this.$route` 和 `this.$router` 同样可以使用。
 
+## 内容摘抄
+
+如果一个 markdown 文件中有一个 `<!-- more -->` 注释，则该注释之前的内容会被抓取并暴露在 `$page.excerpt` 属性中。如果你在开发一个博客主题，你可以用这个属性来渲染一个带摘抄的文章列表。
+
 ## 获取渲染内容
 
 当前的 `.md` 文件渲染的内容，可以作为一个独特的全局组件 `<Content/>` 来使用，你可能想要它显示在页面中的某个地方。一个最简单的主题，可以是一个唯一的 `Layout.vue` 组件，并包含以下内容：

lib/prepare.js

@@ -187,10 +187,12 @@ async function resolveOptions (sourceDir) {
     if (headers.length) {
       data.headers = headers
     }
-    delete frontmatter.content
     if (Object.keys(frontmatter.data).length) {
       data.frontmatter = frontmatter.data
     }
+    if (frontmatter.excerpt) {
+      data.excerpt = frontmatter.excerpt
+    }
     return data
   }))
 

lib/util/index.js

@@ -48,6 +48,7 @@ exports.parseFrontmatter = content => {
   const toml = require('toml')
 
   return matter(content, {
+    excerpt_separator: '<!-- more -->',
     engines: {
       toml: toml.parse.bind(toml),
       excerpt: false