fix: duplicate description meta (close: #565)

- When user set description via front-matter's meta, description generated will be duplicate which
is not friendly to SEO.

Author: ulivz

lib/app/dataMixin.js

@@ -49,6 +49,11 @@ export default function dataMixin (siteData) {
           : selfTitle || 'VuePress'
       },
       $description () {
+        // #565 hoist description from meta
+        if (this.$page.frontmatter.meta) {
+          const descriptionMeta = this.$page.frontmatter.meta.filter(item => item.name === 'description')[0]
+          if (descriptionMeta) return descriptionMeta.content
+        }
         return this.$page.frontmatter.description || this.$localeConfig.description || this.$site.description || ''
       },
       $lang () {

lib/build.js

@@ -138,7 +138,10 @@ module.exports = async function build (sourceDir, cliOptions = {}) {
     readline.cursorTo(process.stdout, 0)
     process.stdout.write(`Rendering page: ${pagePath}`)
 
-    const pageMeta = renderPageMeta(page.frontmatter && page.frontmatter.meta)
+    // #565 avoid duplicate description
+    const meta = (page.frontmatter && page.frontmatter.meta || []).filter(item => item.name !== 'description')
+    const pageMeta = renderPageMeta(meta)
+
     const context = {
       url: pagePath,
       userHeadTags,