Get by multiple IDs

as like:

```python
>>> db['GQSRTmnGbw6Ssuqyu3KYFs']
{ ... }
>>> db[['GQSRTmnGbw6Ssuqyu3KYFs', 'TAy4Qvq82vJk5Ss5vfSY5f']]
[{...}, {...}]
```

If it works well, we can do this:

```python
>>> db[db.find(...)]
[{...}, {...}]
```
