support filtering variants by evidence

[timodonnell]

John Finnigan writes:

Is it possible to use the functionality of --varlens-variants and --varlens-allele-support tool to produce both the union of two .vcf produced from WES data and then use the new union.vcf reduce the list to identify only those alleles for which the variant alleles is also present at the same locus in a RNASeq .bam?

We currently support the unioning and gathering read evidence but not filtering by read evidence (we have a library function that mostly does it but not exposed in any commandline tool).