Add localePath and getRouteBaseName to Nuxt's app

[RomainLK]

Fix #24 . Vue Mixin through plugins are already too far in the lifecycle to be registered on Nuxt's app. The app is in fact, already created when the plugins are registered.

This PR extracts localePath and getRouteBaseName into factories so as to configure where to get $router, $route, $i18n and registers these 2 methods both in the mixin and on app.

( I don't believe anyone will use switchLocalePath from app... )

[paulgv]

Awesome thank you @RomainLK !

[felixdenoix]

Well in fact the switchLocalePath could be quite useful when trying to detect the user locale in the store nuxtServerInit hook in order to redirect him directly to the page with the correct user locale !
That could work like so

user goes to ==> foo.com
website default locale set to 'en'
user locale set to 'fr'
user redirected to ==> foo.com/fr   (if 'fr' is configured)
(user can switch to an other language afterwards if he wishes)

This way you could avoid a redirection on app loading

  async nuxtServerInit({ commit }, { req, app, redirect }) {
    let reqLocale = req.headers['accept-language']
      ? req.headers['accept-language'].substring(0, 2)
      : 'fr'

    let currentLocale = app.i18n.locale

    console.log('currentLocale', currentLocale)
    console.log('reqLocale', reqLocale)

    if (currentLocale !== reqLocale) {
      app.i18n.locale = reqLocale
      return redirect('/' + reqLocale)  <=== [avoid this redirection]
    }

    (........)
  }

Regarding of this use case don't you think, @RomainLK, that it could be interesting to add switchLocalePathto the app ?