Feat: Add config option to run the built in server in cluster mode and to ignore some files of being added with resource hints

[p1n5u]

First, thank you for this awesome vue-cli plugin!

As I wanted to run the built-in server in cluster mode I've added the config option to be able to accomplish this.
As the dev server does not need clustered mode (and also does not like it 🙈) I've adapted the server.js file to set up the cluster as soon as the config option clustered is set to true and the NODE_ENV equals production. The cluster will create one worker per CPU core and also listens to the workers exit events to be able to fork a new one if one crashes. This prevents the server to lose all of its workers in case of failure. The workers will start the server as it was before.
If one of both rules does not apply, the server will launch as it was before my changes.

EDIT 1:
I've also adapted the check isProd in lib/webpack.js. Before my change, the check was based on the VUE_CLI_MODE. But as many developers have multiple custom Vue modes (in my case production, staging, and development) that all build and serve the app in the same way but with different Vue specific environment variables, I've changed the check to use the NODE_ENV. This allows the developers to have custom Vue modes and to control the build/serve process with the NODE_ENV.

EDIT 2:
As I needed the possibility to exclude some static assets of being added to the manifests and being prefetched/preloaded I've added 2 options to specify files from being ignored accordingly to https://ssr.vuejs.org/guide/build-config.html#client-config

[Akryum]

👍