Eased up references regex matching

I found a need to be able to specify `(` for references when using [rework-shade](https://github.com/james2doyle/rework-shade) so this became possible:

   li {
     background-color: blue;
     background: linear-gradient(top, lighten(@background-color, 20%), darken(@background-color, 20%))
   }


I didn't include tests because I wasn't sure how to test this in stock-css (I couldn't think of a time you'd have a property right after a `(`). I wasn't sure if you were okay with having non-stock css in the expected output fixtures. ie. I could write `references.out.css`:

    li {
     background-color: blue;
     background: linear-gradient(top, lighten(blue, 20%), darken(blue, 20%))
   }

But I wasn't sure if that would feel dirty to you.

I also added the ability for a `,` leading character (in attempt to avoid running into this problem on something where it's an arg into a function in the future).