The first image ID can be a middle number in a merged JSON file

[tak-sakumoto]

Hello.

As mentioned in #64 (comment), the first image_id in a merged JSON file get higher if it starts from a middle number defined in the first given file.
Since image_ids in the second file will be rewritten to new numbers, it may not need to keep those middle number any longer.
In other words, there is an approach to start image_id from a base number (e.g. 0 as default).

Please kindly consider this matter. Now I interested to contribute.

Regards,