fix($core): css cannot be extracted together (close: #977)

This is because mini-css-extract-plugin had a internal change at webpack-contrib/mini-css-extract-plugin#265 which will cause issue to VuePress.

Author: ulivz

package.json

@@ -71,7 +71,7 @@
     "markdown-it-container": "^2.0.0",
     "markdown-it-emoji": "^1.4.0",
     "markdown-it-table-of-contents": "^0.4.0",
-    "mini-css-extract-plugin": "^0.4.1",
+    "mini-css-extract-plugin": "0.4.1",
     "nprogress": "^0.2.0",
     "optimize-css-assets-webpack-plugin": "^4.0.0",
     "portfinder": "^1.0.13",