Handle multiple values appropriately

At the moment, this module just validates a single value and doesn't handle multiple values very well. For example, in the output module a couple of tests for the `initial-letter` property are failing, because the module doesn't take into account the position of each of the values.

```
normal | [ ?]
```

https://developer.mozilla.org/en-US/docs/Web/CSS/initial-letter

So, we need to make the module aware of multiple value syntaxes such as this one; test fixtures should be generated to handle cases such as `initial-letter: 3.0 2` for example.