Implement SCSS registry, extendable with ZCML

@ninfaj I've implemented a SCSS file registry for easily extending the SCSS with custom files.

New files can now be easily registered:

```xml
<configure
    xmlns:theme="http://namespaces.zope.org/plonetheme.onegov">

    <include package="plonetheme.onegov" />
    <theme:add_scss path="resources/custom.scss" />

</configure>
```

Files can also be restricted to a specific context (`context` attribute) or a request layer (`request` attribute) or inserted earlier (`before` attribute).
With `remove_scss` it is also possible to disable files (in general, for specific contexts and / or for specific requests).