Support Src Imports

[hrysd]

This PR is to support Src Imports in .vue

example code

const parser = require('vue-parser')

const content = `<template>
  <div class='hoge'>
  </div>
</template>
<script lang='ts' src='./index.ts'></script>
`;

console.log(parser.parse(content, 'script', {lang: ['ts']}, {emptyExport: true}))

Before

// tslint:disable
//
//
//
                                             // tslint:enable

After

// tslint:disable
//
//
//
export { default } from './index'

[ktsn]

Hello, @prograhammer !

Could you take a look this PR so that we can move forward vue-cli v3 RC process by solving TypeStrong/fork-ts-checker-webpack-plugin#111 and vuejs/vue-cli#1104?