sfn create/update

[henry.f...@illumio.com]

Howdy,

My team has been working on moving from cfndsl to sparkleformation for our continuous delivery kind of things, and we've grown a couple of different ways to do "if exists; sfn update $stack; else sfn create $stack; end" and it feels like we're missing something. Is everyone else writing little shims, or is there a flag I'm missing? Would you take a PR?

Cheers,