In my opinion, it's not intuitive that sequence of key_names is mapped to sequence of column_names...
How about this configuration instead?:

```
# no column_names option
# no key_names option

# column name mappings
<column mycolumn1>
  key mykey1
</column>
<column mycolumn2>
  key mykey2
</column>
<column mycolumn3>
  ignore  # don't set value even if mycolumn3 key exists
</column>
<column created_at>
  time_column  # set time to this column
</column>

# these are just ideas for the future
<column mykey>
  default null  # we can set default values if config file parser supports literals
</column>
```