override defaultLang

[cj]

It would be nice to be able to override https://github.com/vuejs/vue-loader/blob/master/lib/loader.js#L30-L34 in the vue settings.

[JounQin]

@yyx990803 I've tried to implement it in pr#500

[yyx990803]

Closing outdated issues - if your issue still persists in the latest version of vue-loader, please file new one using the issue helper, thank you!