1. Modified vcf_parse to update rlen only when an END INFO tag is seen -

Issue: https://github.com/broadinstitute/gamgee/issues/191
1. Modified vcf_parse to update rlen only when an END INFO tag is seen -the END tag is NOT encoded into v->shared.s
2. Modified vcf_write to add END INFO tag if required

@eitanbanks  question about the second condition: "if there is more than 1 alt allele and 1 of them is symbolic (but not <NON-REF> or <X>), then an END signifies a block."
If there is more than 1 alt allele and one of them is symbolic, is it required to have an END INFO field? Another way of framing the question is, can there be a symbolic alt allele for a single location VCF record? I am assuming yes, but just want to confirm.