Add a plain server mode

[srid]

We have two modes right now:

• live server
• static gen

I propose to add third mode:

• plain server

This is like the live-server mode, but there's no websocket involved. The server will serve the HTML straight, and the client will behave similar to the generated static site.

cf. #88 (comment)

The goal here is to be able to host an Ema site behind a proxy (perhaps oauth protected), whilst also enabling it to monitor changes to local files. It'd be like the static site, but live monitoring of the source. And like the "live" version of the site but sans websocket stuff. Assuming git pull is run in the background, the plain server basically gives you a the latest rendered version of the site without having to use Github Pages.