'headless' flet web mode

[hololeo]

Please Describe The Problem To Be Solved

Presently launching flet for web/html, launches the url in a browser tab. This is great for development but not necessary in production.

(Optional): Suggest A Solution

Add commandline switch and / or api option to only serve webpage/websocket without launching browser

perhaps something like

flet.app(target=main, view=flet.WEB_BROWSER mode=headless)

or 

flet.app(target=main, view=flet.WEB_BROWSER mode=serveronly)