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

Thanks
@MauricioCarneiro @jmthibault79 @droazen 
Based on Eric's answer, it seems the only way of knowing whether the VCF record has an END field is to add a boolean flag in the bcf1_t structure: m_has_end_tag.
Reason: if we have a symbolic allele, there is no way of determining whether an END field should be appended simply from rlen, allele string value, allele type and number of alleles