Allow custom VCF filters to be specified via arguments

[selkamand]

We already have pass_only and exclude_sex_chromosomes arguments for identifying which parts of a VCF to include / exclude.

We should also include an argument additional_filters which expects a list of functions - each function should expect a VCF-class object and return a logical vector with 1 element per variant in the VCF. If the function evaluates to TRUE, the variant entry will be kept, and if not it will be excluded

For info on the VCF class object, run

?VariantAnnotation::`VCF-class`