feat(cli): add ability to add vuex and vue-router internal plugins (#1202)

[dhensche]

custom handling for vuex and router when invoking vue add to simply add the dependencies to package.json and point the user to documentation about integrating the libraries into their app

[Akryum]

I think it would be better to extract the vue-router and vuex generator logics from cli-service into their own builtin plugins.

[dhensche]

@Akryum are you thinking they should be separate packages or some sort of new internal plugin structure? It sounded like the desire was to not create new packages for vuex/vue-router #1202 (comment) and they should be treated differently than other plugins

[yyx990803]

So the idea here is:

1. Split out the router/vuex parts inside cli-service/generator／index.js into separate files. Each of them can be a "sub generator" that exposes the same (api, options) => {} signature, and the main generator will just require and call them.

2. Extract the logic of invoking a required generator module into an exported function in cli/lib/invoke.js

3. In cli/lib/add.js when router or vuex special cases are encountered, require the sub-generators from cli-service and invoke them using the function extracted in (2).

[Akryum]

Having builtin plugins would allow reusing the same UI for installing plugins. Or else we will need to work on specific UI for this.