Add a middleware

I don't know if this is a common issue, but right now I needed to start a DoneJS app, and I need to add **one little middleware** to the project. (in this case an custom-auth middleware)

So, currently, to make that work, and still enjoy the benefits of the live-reloading, server-rendering, SPA'ing `done-serve` stuff, I basically had to copy-pasta all the `done-serve` code into a `server.js` file, and run that instead. Not horrible, but I feel it would be a lot _cooler_ if I could just set some config or something `donejs add middleware my-middleware` and have it work out for me.

I know it doesn't have this right now but...


Start by comparing the existing done-serve behavior with the server.js copy-pasta setup described in the issue. Define how a custom-auth middleware would be configured or added while preserving live reloading, server rendering, and SPA support; done is a documented, working extension path rather than a separate copied server.

Written by the indexing model from the issue text.