Make PlaceholderAPI optional

[Gertius]

Description

This PR makes PlaceholderAPI optional.

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

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

Make sure your PR fulfills these requirements:

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

Other Information

N/A

Checklist

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

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

[Jakubk15]

PlaceholderAPI is now optional out-of-the-box. If it is not installed, the plugin will print this warning in console:
"PlaceholderAPI is not installed or not enabled. PlaceholderAPI placeholders will not be available."