Do not format values with slash on custom lostgrid properties

[dmi3y]

This is not really PR but feature question with code example in form of PR to outline the problem.

I'm using lostgrid framework for defining grids. And some of its properties exactly lost-column and lost-row can have a slash separated values like so:

div {
  lost-column: 1/3;
}

And when it processed through the stylefmt the value becomes space separated lost-column: 1 / 3; which is not valid value for lostgrid.

I'm not sure if it's really a concern for stylefmt to take care of not standard properties, or what would be the best approach to make it work if there is any.

Thanks.