Inline Comments

I had always assumed that CSSTree would place comments in the AST according to where they fell in the document, and was a little surprised when I discovered this wasn't the case.

I wasn't sure whether the comments were left out-of-band for a reason.

In any case, I've made a change that would bring them inline, submitted for your consideration. Regardless of whether you decide to merge this PR, or attack this your own way, it'd be nice for comments to be built into the AST like any other construct in the document.

The test suite passes locally without issue.