Vue files support

[fregante]

I saw a handful of Vue-related issues. It would be good to:

• document how to add vue
• add tests for vue files to avoid regressions

Better yet:

• add native support

[simwai]

This would be really cool. I had also the problem, that I couldn't use it with Vue in VS Code with the official xo extension. It said xo extension wouldn't support Vue.