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

That would be pretty cool, yeah.

Actually I see that <X> is already handled as VCF_OTHER in vcf.c:bcf_set_variant_type(), interesting.  Not sure if that suits us or if VCF_NON_REF is needed.  Probably better to split this off into a separate issue anyway.