Vue loader v15 changes

[joshrosenhanst]

Vue-loader recently updated to v15 and has some relevant breaking changes: https://vue-loader.vuejs.org/migrating.html#notable-breaking-changes

• Vue Loader v15 now requires an accompanying webpack plugin to function properly
• Vue Loader v15 now uses a different strategy to infer loaders to use for language blocks. Essentially language blocks like <style lang="sass"> will be treated as if a .sass file was loaded.

V15 is the version installed when you run the npm install command listed in the README.

This PR should fix the webpack.config.js support for vue-loader v15 and updates the README to reflect the new webpack config.

[msftclas]

All CLA requirements met.

[orta]

I don't think. we need this now that https://github.com/microsoft/TypeScript-Vue-Starter/pull/47 is in... Right?

[joshrosenhanst]

I don't think. we need this now that #47 is in... Right?

Yup that looks like it covers this issue. Thanks