[1222] Add a WebM and MP4 loader (file-loader) to the default build webpack config

I'm meeting the case that need to use video source in my page.

When the loaders are defined in the nuxt.config.js, the default loaders will be overwritten., the docs says.

Meanwhile I'm thinking that why not use delta updates. I mean the loader config is made up of user's and default config. I don't want add a loader my want to overwrite all config.