text: Title: Allow setting of alias along with version, for use with rollback
  Body: ## Context
  AWS lambdas allow you to set an `alias` along with `versions`, per the [documentation](https://docs.aws.amazon.com/lambda/latest/dg/aliases-intro.html). Although this may not be useful within the zappa_settings, having a switch like `--alias` during zappa deploy could allow a user to set this field, and reference said alias during a rollback. This could also allow for other useful features, like setting a `default` rollback, if a function fails, but for now, just being able to create the references would be useful.
  ## Use case
  For our projects, I have been using AWS tags to create a tag for the function, setting it to the most current git commit hash, so we can compare the latest commit to the currently deployed commit. It allows us to reference it so that we can directly deploy any previous commit, without being tied to 'how many versions before'. Ideally, setting the aliases could be a better way of handling this use case.
  ## Optional use case
  Regarding this use case, (this would be terribly specific), it could be useful to have aliases set by default to git commit hashes, so they could be referenced, and allow a different type of hash or naming mechanism in zappa_settings. Thus, we could rollback to specific commits by referencing aliases, while the 'versions back' ability would still remain.