Enable skip-delete by default

[danuker]

Thank you for creating this tool! But I seem to have stumbled into a "papercut" for newbies.

Here is a suggestion for a change. I hope you find it worthwhile.

User Behaviour

1. Do not read the docs
2. Use labeler without configuring it, expecting that tags are added

Expected behaviour

The default behavior should not delete tags. They are precious information, and on repositories with thousands of tickets, they are difficult to restore.

Actual behaviour

skip-delete is false by default, leading to possibly unintended deletion of tags.