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

Insertion single location record:
Reference: T      Alt: TAG
length(ref) = 1
length(alt_allele) = 3
rlen = 1
The condition is satisfied but it is not a block.
Can a deletion have length(reference) == 1? I cannot find such an example in the VCF format.