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

The code in vcf_parse looks like this:
if ( END exists in INFO field)
     rlen = END - pos;
else
    rlen = strlen(allele[0]);   //reference length
The logic in vcf_format assuming the END tag is eliminated from bcf1_t
if(rlen != strlen(allele[0]))         //seems easiest case
      add_END();
else          //rlen == strlen(ref)
{
   ?  //hard part - I cannot think of clear conditions now  
}