Adding an additional session
I would like to introduce a secure session in our app changing the existing and behaviour of `this.session`.  So, for example I might add `this.secure_session` and provide my own encode and decode methods.

Could this be as simple as adding a context_name configuration?

``` javascript
const session = require('koa-session')
const cryptoSession = require('koa-crypto-session')

app.use(session({context_name: 'session'}, app));
app.use(cryptoSession({context_name: 'secure_session'}, app));

// this.session
// this.secure_session
```

I found two references to 'session' .. `__defineGetter__` and `__defineSetter__`
