Enhancement: Add Option to Filter by Position or Specific Variants

[berntpopp]

Add an option to filter by genomic position or specific variants instead of gene names. This will enhance the flexibility of the script, allowing users to target specific regions or variants more precisely.

Acceptance Criteria:

• Implement filtering by position or specific variants.
• Validate the reference allele when filtering for variants.
• Update the documentation to describe the new filtering options.

[berntpopp]

Duplicate of #14