Feature request: option to bypass `maxSnack`

[simon-abbott]

This project is awesome, and has saved me a ton of time setting up notifications in a webapp I'm working on, but there's one small quirk I noticed. I set up an "is the browser offline" listener that displays a persistent alert if the answer is yes. It works great, but it eats up one of the maxSnack slots even though I'm setting it to persistent and displaying it with a different anchor origin. It would be great if there was a way to tell enqueueSnackbar "hey, don't count this one towards the limit". I don't think it's likely to be an actual problem in practice since we're usually only going to have one normal alert at a time anyway, but it still seems like a missing feature from this otherwise amazingly customizable package.