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

My understanding from that conversation is that the case with multiple alt alleles including `<X>` or `<NON_REF>` will never be a block with an END tag: it will only be an event at a single location.  Is that correct?