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

@kgururaj is this code from `vcf_parse` implemented in htslib today? 

That looks simple enough actually. And I like it.

I think that we can skip the else because right now we wouldn't know what to do if `rlen == allele[0]` but this was a ref block anyway. 

Can you push this change? We can merge it in and experiment with it.