Support negative values in box-shadow string parsing

[emattias]

Without this you get unable to parse color error when animating box-shadow that has negative values. For example:

      boxShadow(sprite, {
        from:
          'rgba(0, 0, 0, 0.1) 0px 18px 12px -12px, rgba(0, 0, 0, 0.1) 0px 5px 23px 10px',
      });

[ef4]

Thanks.

There is already pretty good test infrastructure for this here if you wouldn't mind adding some examples to cover this.

While we're at it, I suppose the spec probably allows decimals too. We'd better check what the precise grammar is.