code: function getFilePathFromLine(bundleFile, line, variables) {
  	// handle variables
  	var varRegex = /@{([^}]+)}/;
  	var match;
  	while (match = line.match(varRegex)) {
  		var varName = match[1];
  		if (!variables || typeof (variables[varName]) === 'undefined')
  			throw new gutil.PluginError(pluginName, bundleFile.path + ': variable "' + varName + '" is not specified');
  
  		var varValue = variables[varName];
  		line = line.substr(0, match.index) + varValue + line.substr(match.index + match[0].length);
  	}
  
  	if (line === '')
  		return null;
  	
  	// handle `!` negated file paths
  	var negative = line[0] === '!';
  	if (negative)
  		line = line.substr(1);
  
  	// get file path for line
  	var filePath;
  	if (line.indexOf('./') === 0)
  		filePath = path.resolve(bundleFile.cwd, line);
  	else
  		filePath = path.resolve(bundleFile.base, line);
  	
  	// return path
  	if (negative)
  		return '!' + filePath;
  	return filePath;
  }
docs: parses file path from line in bundle file