specify the version to install

In the CLI:

```
ffcss use materialfox@88.0
```

`latest` can be used to refer to the latest tag.

(default behavior is to get the latest commit)

For themes that aren't hosted as git repositories, the `urls.version` key can be used, where `{{ version }}` will be replaced by the current version.

The URL should be able to handle `latest` as a version.