Pluggable storage layers
Hey, I needed to be able to use Mongo for the session back-end so I went ahead and wrote my own session middleware for this. As it turns out I ended up using most of koa-session and what I have works almost exactly the same (and even uses the same tests):

[https://github.com/hiddentao/koa-session-store](https://github.com/hiddentao/koa-session-store)

It defaults to storing in the cookie but you can supply a 'layer object' instead. I've written a MongoDB layer - [koa-session-mongo](https://github.com/hiddentao/koa-session-mongo) - which is based on the connect-mongo module.

If you like it then I'd be more than happy to fork koa-session and raise a pull request with all the changes. At the very least it would make the naming better:
- koa-session - the core module
- koa-session-\* - a specific storage layer
