Feature: add option keepDefaultRouter

[IlyaSemenov]

In my project, I want to keep pages/ directory but I want to customize routes per request.

My real-life use case is:

https://myproject.com/*** should come from pages/*.vue
https://***.myproject.com/*** (subdomains) should come from pages/app/*.vue (entirely different set of pages)

Unfortunately, Nuxt.js lib/app/index.js and lib/app/router.js don't provide any suitable hooks.

If I use @nuxtjs/router with fully custom routes, I lose the pages/ directory parser, scrollBehavior and possibly other goodies. I am happy to use the default routes generator, I just want to have some post-processing.

This PR is an attempt to solve the problem. How it works: if @nuxtjs/router module is created with { keepDefaultRouter: true } (false by default), the default router will not be thrown away but kept in .nuxt/defaultRouter.js. Custom router.js may then import it and reuse its options when creating a new Router instance, then amend, filter or modify routes or other router options. I included a simple unit test that shows how it may work (it's more complicated in real life, of course).

---

This solution is surely not perfect. Instantiating default router simply to access its options is suboptimal (it would rather be better if Nuxt's lib/app/router.js exported not only createRouter but also routerOptions). Filtering and modifying rules per request is also suboptimal, in my case I only need two static subsets and it would be great if I could emit them with the builder rather than do that in runtime for each request. But at least it's something.

[Atinux]

Hey @IlyaSemenov

Sorry for the delay, great idea BTW, thanks for adding the tests as well!