add an optional filter to VCF variant annotation

[julie-sullivan]

Added a parameter to variant-annotation to filter on the FILTER column in VCF files:

      --filter                     STRING        string indicating the FILTER label that variants must have to be annotated. Only variants with this label will be written in the output.

• Added an optional FILTER option
• Added a passing unit test (all tests are passing for this test class)

Please see the PR to merge in v4 #554