View media query stats when in dev mode

[igemnace]

It would be useful to have a tiny overlay that displays media query stats (e.g. wXS: false, wXM: false) in the corner, similar to how Chromium shows a tiny overlay that displays the resolution if the window is resized while Chrome Dev Tools is open.

Such a feature should definitely only be active in dev mode. Maybe a dev option, so one can easily do:

// ...

Vue.use(VueMediaQueryMixin, {
  dev: process.env.NODE_ENV === 'development',

  // ...
});

or whatever condition is needed to specify dev mode.