Allow every and schedule to take array of job names

Just seen that for my case,

```js
agenga.every('5 minutes', ['job1', 'job2', job3']);
```

is a bit better than,

```js
agenda.every('5 minutes', 'job1');
agenda.every('5 minutes', 'job2');
agenda.every('5 minutes', 'job3');
```

I understand it's interface change and there are a lot of reasons to reject it. But if you see this is OK and merge upstream I'll update documentation as well :)