Named format uses `Vec<String>` instead of `&[String]`

Currently, `AmethystNamedFormat` takes extra options in the form `Vec`. This is suboptimal since it requires moving the whole container and cloning it if it needs to be reused. Instead it could be `&[String]`, or even `&[S]` with `AmethystNamedFormat where S: Deref`.

I'm happy to write up a PR for this if people think it's a good idea.