[78] Dash in layout filenames

If layout component file name contains special characters, this leads to incorrect App.vue generation: as an example: (layouts/docs-components.vue)

let layouts = {
  _docs-components: process.BROWSER_BUILD ? () => System.import('...docs-components.vue') : require('...docs-components.vue'),
}

This throws syntax errors. keys can be easily quated to avoid this problem.