Improve compilation speed for development builds

[Ruin0x11]

1. Use watchify for incremental builds instead of naive polling (npm run watch). Speeds up the time between changes by ~17x.
2. When running npm run watch, set the environment as development, and open a websocket for live reloading. The event for live reloading is sent when any template/CSS/JS is changed. Can be disabled by passing --no-live-reload to build.js.
3. Better formatting of errors during incremental compilation.

[neobooru]

Looks good.
I do have a small thing to nitpick about though. When you build the site for production (aka simple npm run build) the live-reloading code is still embedded into the app. I know something like webpack could remove this, but we aren't using that at the moment.
I assume it would be rather hard to remove those lines from the production build using our current build system, but I'm just asking to make sure.

[Ruin0x11]

Maybe we should create a template that holds main.js and autogenerate it like with the HTML templates? It might make the code for that less clean though.

[neobooru]

Thanks for your work 👍
I squashed it to avoid adding the watch.sh to our history.