Add a CLI flag to automatically add the future flags to the simple config stub

[dextermb]

As mentioned in my previous PR (#2372) I'd love to see automated future flags adding to the simple config stub, so I thought I'd give it a shot.

I've implemented the --future flag on tailwind init that will add the future flags to the simple config stub. Seems to work well and is fairly self contained. I'd appreciate feedback on my PR.

#2379.