While developing this, I noticed that the type was handling inconsistently the
expectation that a cdist object with the same __object_id gets *modified*.
Instead more and more lines were added to, e.g. SigningTable and KeyTable.
In order to solve this, some backwards compatibility breaking is necessary.
This is probably not too terrible since:
- the `--selector` parameter was mandatory, therefore the fallback for the key
location is triggered.
- OpenDKIM uses the first match in `SigningTable` and `KeyTable`
- __line and __block respectively append if they do not match
Closes#19 and #20.
