Allow whitespace before single-line comments

useful for LESS/SCSS which allow scoped selectors and namespaces

For example:

    // # Util mixins
    // Various mixins that are sprinkled into other selectors
    #utils {
      
      // ## Page Breaks
      #pb {

        // Break before the current element
        .before(@avoid) {
          /* Some rules ... */
        }
      }  
    }