Support characters like `$` in less code

[freewind]

Current jless throws exception for such code:\

.title {
     width: ${width}px;
}

Because it doesn't allow $ and {}.

I think it's good to support them, for example, I can use jless in a html page or a haml page with custom filter:

 :jless
      .title {
            width: ${width}px;
      }