Added support for onBadUrl

[delvedor]

Now find-my-way offers the support for a separate handler in case of a bad url. If it's not configured, the behavior will be the same as before, which means calling the defaultRoute in case of the lookup and returning null in case of find.

Usage:

const router = require('find-my-way')({
  onBadUrl: (path, req, res) => {
    res.statusCode = 400
    res.end(`Bad path: ${path}`)
  }
})

Related: fastify/fastify#1884

[mcollina]

LGTM

[Eomm]

Great! Thanks!