`zpa publish` (work in progress)

I just wanted to post this to show roughly how `zpm publish` could work. Please give it a try and let me know what you think.

To make this work, you need to bundle and deploy [this zapp](https://github.com/larsbutler/zpa) to the `zpa_publish` container.

I haven't yet tested to see if this works across different user accounts. I'm guessing it won't, so we'll have to figure that out.