feat: add on_base inclusion

[seeplusplus]

Resolves #225.

• If no on_base is specified in delete-merged-branch-config.yml then everything operates normally (i.e., we delete branches as long as they are not explicitly excluded).
• If base is in on_base and head is not in exclude then we delete head.
• If base is in on_base and head is in exclude we do not delete head.
• If on_base is non-empty and base is not included, we do not delete head.

[cnb-philip]

Is this going to be released at any point in the near future? We need this feature before we can use.

[SvanBoxel]

Thanks for the reminder. I just created a new release: https://github.com/SvanBoxel/delete-merged-branch/releases/tag/1.4.2