Fix parsing of comments within values about css

[lydell]

I’ve started looking at this … and while I did so, I found that the solution I envision easily could allow us to preserve such comments. I’m thinking that each node should get a raw property, which is an object. The raw object duplicates all of the properties of the node that are strings or arrays of strings and which had comments stripped from them. The duplicates in the raw object are not stripped from comments. Then css.stringify could do something like value = (node.raw.prop && stripComments(node.raw.prop) === node.prop ? node.raw.prop : node.prop); this.emit("foo" + value)—that is, use the raw value if the cleaned value hasn’t been changed. The compress compiler could ignore that, though (or perhaps allowing certain “hack” comments). What do you think about that?