Improves vue-page-title reactivity when using vue-router

[ricardocastanho]

Improves vue-page-title reactivity when using vue-router

Now the route are watched and when it changes the title function are re-called.

• Improves vue-page-title reactivity when using vue-router.
• Add vue-router usage tests.
• Update docs.

<script>
export default {
  title () {
    return `My title is: ${this.$route.query.foo}`
  }
}
</script>