chore: upgrade mini-css-extract-plugin and lock version

This commit fixed #977 for 0.x.

Author: ulivz

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

@@ -271,7 +271,9 @@ module.exports = function createBaseConfig ({
         styles: {
           name: 'styles',
           // necessary to ensure async chunks are also extracted
-          test: m => /css-extract/.test(m.type),
+          test: m => {
+            return /css\/mini-extract/.test(m.type)
+          },
           chunks: 'all',
           enforce: true
         }

packages/@vuepress/core/package.json

@@ -51,7 +51,7 @@
     "koa-range": "^0.3.0",
     "koa-static": "^4.0.2",
     "lru-cache": "^4.1.2",
-    "mini-css-extract-plugin": "^0.4.1",
+    "mini-css-extract-plugin": "0.4.4",
     "optimize-css-assets-webpack-plugin": "^4.0.0",
     "portfinder": "^1.0.13",
     "postcss-loader": "^2.1.5",