add koa middleware support.

[NoobTW]

I use koa instead of express now, so I add koa middleware for koa support.

[EmilyQiRabbit]

Need that!!!!! Why not merge it to master???

[NoobTW]

@EmilyQiRabbit maybe this helps?

[EmilyQiRabbit]

I referred to your code and solved my problems🙋💯But if it can be used as a middleware, will be better 🌸 And I wanted to solve the problems using imported package~So I didn't consider koa-json2xlsx 😜