code: function createObservable (cfg) {
      const {dir, taskSet, tasksToRun, watchMode, spawn, env, log} = cfg;
  
      const tasks = tasksToRun.map(taskName => {
          const commandList = taskSet[taskName];
          if (!commandList)
              throw new Error (`No such task name: ${taskName} was configured`);
          return commandList;
      });
  
      return createParallellCommandsExecutor (
          tasks,
          dir,
          env,
          watchMode,
          {spawn, log});
  }
docs: Create a build-task executer as an Observable
  
  @param cfg {{
  dir: string,
  taskSet: Object.<string, string[]>,
  tasksToRun: string[],
  watchMode: boolean,
  spawn: Function,
  env: Object,
  log: Function
  }} Configuration to execute
  @return Observable