Add support for nested routes

[paulrobertlloyd]

With express.Router, it’s possible to create self-contained routing beneath a path set in a parent router. Using an example slightly modified from the Express JS documentation:

// birds.js
import express from 'express'

const router = express.Router()

router.get('/', (req, res) => {
  res.send('Birds home page')
})

router.get('/about', (req, res) => {
  res.send('About birds')
})

export const birdRoutes = router

// routes.js
import { birdRoutes } from 'birds.js'

app.use('/birds', birdRoutes)

The app will now be able to handle requests to /birds and /birds/about.

However, while this library looks for paths that match req.originalUrl, it only creates next and previous paths based on the route handler. For example, given the following journey:

const journey = {
  '/': {},
  '/about': {}
  '/species': {}
}

Upon visiting /about, the following paths object is provided:

{
  back: '/',
  current: '/birds/about',
  next: '/species'
}

Note that back and next values don’t contain the parent path segment. Clicking on links containing those values will return the wrong (or no) page.

---

This PR appends req.baseUrl which includes this information. With this fix in place, the following paths are generated:

{
  back: '/birds',
  current: '/birds/about',
  next: '/birds/species'
}

If there is no req.baseUrl, the previous behaviour is retained.