Integrate locale in the routing

[rayfranco]

Being able to link a routing parameter to the current locale:

/:lang/hello/world

I came across several problems to achieve this in a nice way. See the following issues:

https://github.com/kazupon/vue-i18n/issues/65

https://github.com/vuejs/vue-router/issues/745

https://github.com/vuejs/vue-router/issues/738

Any idea is welcome.