Add setting to conditionally change custom repositories behavior

[maksimkurb]

Fixes #29, #30

This PR introduces BPAPT_CUSTOM_REPOS, allowing users to control how custom repositories are added to the APT sources list. It supports three options:

• append (default) – Custom repositories are added at the end of sources.list.
• prepend – Custom repositories are added at the beginning of sources.list.
• custom-only – Only custom repositories are included, omitting defaults.

This maintains the existing behavior by default while providing more flexibility.