Support Koa

[satazor]

https://github.com/webpack/webpack-dev-middleware supports express and koa. We should do the same.

[tadjohnston]

Any updates on this?

[satazor]

This is a very low priority issue for us. If you are willing to make it work with koa as well, I will review the PR and merge it.

[tadjohnston]

Sounds good @satazor! If I can get some time, I will definitely give it a shot, thanks!