Support for nested items

[lemieux]

This allows value like -webkit-image-set(url('some-url.png') 1x, url('some-other-url.png') 2x) to be parsed and transformed the by loader.

It fixes #200.

[sokra]

For what reason is isValue tracked?

[lemieux]

maybe it should be named differently, but it's basically to reuse the same processNode logic on nested properties... should probably be named isNested

[sokra]

please fix the spacing. It should be tabs.

[sokra]

isValue is always true here, because a nested item has this representation:

{
    type: "nested-item",
    name: name,
    nodes: [
        { type: "value", nodes: [...] }
    ]
}

So I think you could remove this check here and always use the true case.

[lemieux]

using the true case makes the background img 2 test and the font face tests fail.

[lemieux]

If I remember correctly, it was because in the case where it's processing a value, the urls in it were not transformed.