Set render functions in Constructor's extendOptions

[HerringtonDarkholme]

Fix #432 and vuejs/vue#4052

When users have Vue.mixin(globalMixin), vue-loader will fail for constructor style export because Ctor.options is replaced and redner function lost.

In vue's core code. resolveCtorOptions will compute the options for a consturctor by merging extendOptions and SuperOptions. Vue.mixin will reset SuperOptions and constructor's option is replaced. So render function which vue-loader set is lost. However, if extendOptions has render, newly created options will preserve render function.

[yyx990803]

Thanks, I think it's better to fix it in core so that vue-loader's bundle output will be smaller.

See vuejs/vue@09f9783