fix: missing space between italics and bold (close: #661)

"markdown-it" will intendedly reserve this space, and this issue was caused by "preserveWhitespace: true" (See: https://github.com/vuejs/vue/tree/dev/packages/vue-template-compiler#options),

Impact on performance (Size of whole assets): Before (1952K), After (1960K), which is an acceptable change.

Author: ulivz

lib/webpack/createBaseConfig.js

@@ -108,7 +108,7 @@ module.exports = function createBaseConfig ({
         .loader('vue-loader')
         .options({
           compilerOptions: {
-            preserveWhitespace: false
+            preserveWhitespace: true
           },
           cacheDirectory,
           cacheIdentifier