fix($build): markdown config should be included in cache identifier

It's reported by #664, but #664 cannot be fixed fully since watching webpack config changes shouldn't be handled in VuePresss

VuePress can only ensure that when user changes the markdown config, the cache identifier will be updated, so user needn't to clean the ./node_modules/.cache manually.

See: webpack/webpack#3153, webpack/webpack-cli#15

lib/webpack/createBaseConfig.js

@@ -88,6 +88,7 @@ module.exports = function createBaseConfig ({
     isProd,
     isServer,
     config: (
+      (siteConfig.markdown ? JSON.stringify(siteConfig.markdown) : '') +
       (siteConfig.chainWebpack || '').toString() +
       (siteConfig.configureWebpack || '').toString()
     )