Make ICssFunctionValue public

## New Feature Proposal

### Description

I'm trying to migrate to latest version and finally try to implement urls extraction from css without changes to AngleSharp itself (see #10). Looks like only missing part is unavailability of `ICssFunctionValue`  interface, implemented by `UrlReference`

### Background

I need to find all url's in css file, specified using 'url()' function.

