Add --open flag

[dansteren]

It would be nice if you could run zwitterion --open (or something similar) and it would automatically open a browser window to localhost:8000 or wherever the site is being served. Better yet, have this be the default behavior.

[lastmjs]

Sounds reasonable to add an option for this.

[lastmjs]

I don't think it should be default behavior, because I think that would be unexpected for something that is supposed to just be a static http server, and I personally use Zwitterion and other http servers in contexts that have nothing to do with a browser. Zwitterion uses itself as a static server to do its static build, for example. We wouldn't want that popping up a browser window, so I think an option would be best.