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

Here was my reason: If the bcf1_t structure no longer has an END tag, then rlen is the only field which specifies the length of the interval. However, rlen could be > 1 for single position records specifying a deletion. In vcf_format, to the best of my knowledge, there is no way to determine whether it is a single position VCF record with deletion or a genomic interval for which an END tag should be added. Is there a check that you would suggest?