Support PlaceholderAPI Relational Placeholder

[Gertius]

Description

This PR resolves #99.

What kind of change does this PR introduce? (Check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Build related changes
• [ ] Documentation code
• [ ] Other, please describe:

Make sure your PR fulfills these requirements:

• [x] Tests for the changes have been added (where possible)
• [x] Documentation (README.md, CONTRIBUTING.md) has been updated, if needed
• [x] All new and existing tests have passed

Other Information

N/A

Checklist

By submitting this PR, you confirm that you have completed the following:

• [x] I have read and understand the CONTRIBUTING Guide
• [x] I have checked that no existing PRs fulfill my requirements and appear to be duplicates
• [x] The PR title follows the guidelines defined in the contributing guide
• [x] All tests pass locally with my changes (if applicable)
• [x] I have performed a self-review of my own code