Extending entry

[Akryum]

There are a lot of cases where we need to modify the main.js file (or main.ts). For example importing modules, adding options to the instance and such.

Proposed API:

// Generator
api.extendScript('./src/main', file => {
  // Import a file
  file.import('{ apolloProvider }', './vue-apollo')
  // Direct import
  file.import('./style.css')
  // Add an option to the instance
  file.extendOptions({
    provide: 'apolloProvider.provide()',
  })
})

[yyx990803]

Closing in favor of #845