added salt to cache name + added a public method for fetching variables

1. When deploying our site to production we came across a problem in which the scss files didn't change directly but the registered functions I've defined using the PHP code outputted a different result, causing the scss files to load from cache even though changes were made to them. I've added the `$salt` argument which allows us to add the revision to the `serve` method, thus preventing the load of old cache once the revision of the code has changed. If think that this is a good add-on, so I hope you'll pull this code to your project as well.

2. Sometimes we might need to reach scss variables from within the registered function, so I've created a porxy method `getVariable` for fetching the scss code local variables (same as `get`)