add tests, @media support, js-land vars and support for co-existing globals/namespaced vars

previously it seemed like you had to pass the option to choose global or namespaced, now both are supported. The only thing I think we may want to change later is the var rule definitions to something like:

```css
vars {
  some: globals;
}

vars:colors {
  something: here
}
```

otherwise rules such as `$widget {}` would not substitute properly, it looks like a variable substitution _and_ a variable rule. Not a big deal for me personally, I don't have any use-cases for variables in selectors but hey