Add metadata to VCF-I header

[jdidion]

In the interest of promoting data provenance and reproducibility, I propose to add some headers to the VCF-I spec:

##program=<ID=hap.py, version=0.3.9, commandline=...> ##comparator=<ID=vcfeval, version=3.5.1> ##truthVCF=<ID=GIAB, file=giab_truth.vcf.gz, url=ftp://ftp-trace.ncbi.nlm.nih.gov/giab/ftp/release/NA12878_HG001/> ##queryVCF=<ID=Sample1, file=sample1.vcf.gz, sex=M> ##truthRegions=<ID=GIAB_default, file=giab_truth_regions.bed, url=..., insertionPadded=true> ##targetRegions=<ID=AgilentExome6.0, file=Exome-Agilent_V6.bed, url=https://github.com/AstraZeneca-NGS/reference_data/blob/master/hg19/bed/Exome-Agilent_V6.bed?raw=true>