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

I agree with adding another field, but only if there is no other way.

Last time we talked to Heng (@lh3) about this he disagreed. If I remembered
correctly he mentioned that rlen was already the end field. Maybe he can
help this discussion or when I come back I can try to talk to him in person.