fix: override style issues

1. duplicated generated override style
2. unexpected style order, override style should be at the end of the extracted style bundle. (ref: webpack-contrib/mini-css-extract-plugin#130)

Author: ulivz

lib/app/app.js

@@ -7,6 +7,9 @@ import { siteData } from '@temp/siteData'
 import enhanceApp from '@temp/enhanceApp'
 import themeEnhanceApp from '@temp/themeEnhanceApp'
 
+// generated from user config
+import('@temp/override.styl')
+
 // built-in components
 import Content from './components/Content'
 import OutboundLink from './components/OutboundLink.vue'

lib/default-theme/styles/config.styl

@@ -17,5 +17,3 @@ $MQMobileNarrow = 419px
 
 // code
 $lineNumbersWrapperWidth = 3.5rem
-
-@import '~@temp/override.styl' // generated from user config

lib/webpack/createBaseConfig.js

@@ -49,7 +49,7 @@ module.exports = function createBaseConfig ({
         : path.resolve(__dirname, './noopModule.js'))
       .end()
     .extensions
-      .merge(['.js', '.jsx', '.vue', '.json'])
+      .merge(['.js', '.jsx', '.vue', '.json', '.styl'])
       .end()
     .modules
       // prioritize our own