--open for serve

[hroncok]

Suggested by @frenzymadness:

Add --open for serve (and freeze --serve) to open the served webpage in webbrowser (as Jupyter Noteook does).

[encukou]

Or perhaps even add a dedicated open command -- it's a shortcut, so let's make it short :)

Also, if we're going for convenience, integrating something like livereload would be a good idea.