fix: prioritize frontmatter's title and lang (close: #177)

Author: ulivz

Diff for: lib/app/dataMixin.js

@@ -47,13 +47,13 @@ export default {
       return targetLang || defaultLang || {}
     },
     $title () {
-      return this.$localeConfig.title || this.$site.title || ''
+      return this.$page.frontmatter.title || this.$localeConfig.title || this.$site.title || ''
     },
     $description () {
-      return this.$localeConfig.description || this.$site.description || ''
+      return this.$page.frontmatter.title || this.$localeConfig.description || this.$site.description || ''
     },
     $lang () {
-      return this.$localeConfig.lang || 'en-US'
+      return this.$page.frontmatter.lang || this.$localeConfig.lang || 'en-US'
     },
     $localePath () {
       return this.$localeConfig.path || '/'