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

There is also the possibility of a deletion where `length(ref) == 1 && rlen
> 1` and this is **not** a block.

The correct check here is: `length(ref) == 1 && max( length(all alt
alleles) - length(ref) + 1) != rlen`