koa-router@7 supports generator middleware for compatibility

[hardfist]

node.js version: 8.9.4

npm/yarn and version:

koa-router version: 7.4.0

koa version: 2.5.1

Code sample:

var Koa = require('koa');
var Router = require('koa-router');
var Boom = require('boom');

var app = new Koa();
var router = new Router();
router.get('/demo',function*(next){
   this.body = 'this is a test';
   yield next;
 // not working
});

app.use(router.routes());
app.use(router.allowedMethods({
  throw: true,
  notImplemented: () => new Boom.notImplemented(),
  methodNotAllowed: () => new Boom.methodNotAllowed()
}));

Expected Behavior:

supports generator

I recently update koa@1 to koa@2,since koa@2 supports generator koa@1 for compatiable,I can migrate my app progressively and don't need to update all my generator to async/await middleware,
but koa-router@2 doesn't support generator yet which makes migrations hard。So like koa2 I think if koa-router@2 also supports generator middleware It will be very convenient for migration. I make an pr #467 to support generator. Thanks.

Actual Behavior:

not supports generator

Additional steps, HTTP request details, or to reproduce the behavior or a test case: