Enhancement: Implement a `filterFunction` in `variantLinkerProcessor.js` to allow for flexible filtering of the results before formatting and outputting them.

[berntpopp]

Current Behavior

Currently, the filterFunction in variantLinkerProcessor.js is set to null, and there is no mechanism to filter the results based on specific criteria before formatting and outputting them.

Proposed Solution

1. Define a Filter Function: Implement a default filterFunction that can be customized to filter results based on various criteria.
2. Command-Line Argument: Add a command-line argument to allow users to specify their own filtering criteria.
3. Apply the Filter: Modify the filterAndFormatResults function to apply the filterFunction to the results before formatting.

Acceptance Criteria

• Implement a default filterFunction in variantLinkerProcessor.js.
• Add a command-line argument to allow users to specify custom filtering criteria.
• Ensure the filterAndFormatResults function applies the filterFunction correctly.
• Update the documentation to describe how to use the filtering functionality.
• Add tests to verify the functionality of the filterFunction.

[berntpopp]

Relates to #26