feature request - add `prefix` option to Router constructor

[saadq]

One thing I miss from koa-router is being able to prefix all your routes.

For example:

import Router from 'koa-router'
const router = new Router()

router.get('/api/cars', async ctx => {

})

router.get('/api/bikes', async ctx => {

})

could be simplified to this:

import Router from 'koa-router'
const router = new Router({ prefix: '/api' })

router.get('/cars', async ctx => {

})

router.get('/bikes', async ctx => {

})

This was quite useful when using a webpack dev server where I would proxy all routes starting with /api to my Node server.