Clear Diagnostics on Configuration Changes

[ObliviousHarmony]

All Submissions:

• [x] Have you checked for duplicate PRs?
• [x] Have you added an entry to the CHANGELOG.md file's [Unreleased] section?

Changes proposed in this Pull Request:

Since the output might change as a result of configuration adjustments we should clear the diagnostics. This keeps us from displaying incorrect information.

Closes #72.

How to test the changes in this Pull Request:

1. Open a project and create a diagnostic error.
2. Break the configuration and observe clearing.
3. Change the PHPCS file and observe clearing.