allow multiple instance-setup scripts to be specified

It would be nice if Cloudy allowed multiple instance-setup scripts to be specified at once.

The difficulty here would be combining the cloud-init scripts from the different instance-setup scripts. However, cloud-init provides a way to [combine multiple cloud-init scripts within a MIME multi-part file](https://cloudinit.readthedocs.io/en/latest/explanation/format.html#mime-multi-part-archive). Cloudy could likely make use of this.

It would also be nice if a default instance-setup script could be specified in the `~/.config/cloudy/cloudy.yaml` file, which would be used for _all_ instances. I'm imagining this would do things like install packages that would be desired on _all_ systems, like `neovim`, `tree`, etc.

The user could likely leverage cloud-init's [Jinja templates](https://cloudinit.readthedocs.io/en/latest/explanation/format.html#jinja-template) and [instance variables](https://cloudinit.readthedocs.io/en/latest/explanation/instancedata.html) to write cloud-init scripts that are compatible across distros.

No implementation files or tests are named. Start by locating how Cloudy parses instance-setup options and ~/.config/cloudy/cloudy.yaml, then review the linked cloud-init MIME multipart and Jinja documentation. Done means multiple scripts can be combined for one instance and a configured default script applies to all instances.

Written by the indexing model from the issue text.