I agree that it's good to have consistent format across other plugins for existent users. But I don't think it's intuitive way to use 2 arrays to configure mapping for new users because either `column_names` or `key_names` doesn't include meaning of mapping.

1. I thought following format is very good if fluentd supports hash literals. How do you think?:

```
columns {mycol1: mykey1, mycol2: mykey2}
```

I think it's ok to use 2 arrays so far because fluentd doesn't support hash literals yet.

2. How do users map time of a event to a column?