Proposal: Expose JSON API Routes

## Reason
In some circumstances someone might need to use the available JSON API Routes without needing to mount the entire Express App instance through a middleware or from a standalone Kue instance.

## Proposal
Expose the JSON API Routes, so it could be used as:
`const kueJsonRoutes = require('kue').json`
or through the App instance
`const kueJsonRoutes = require('kue').app.json`

Will like to know what others thoughts are on integrating this. I can open a PR for changes if accepted.

Thanks!

Start by locating how Kue currently mounts its JSON API routes through the Express app. Compare the proposed require('kue').json and require('kue').app.json entry points, then review the surrounding issue discussion for an accepted integration direction. Done means the routes are independently exposed without requiring the full Express app, with the public API and usage agreed.

Written by the indexing model from the issue text.