Allow INFO field name to have underscores

INFO fields with underscores were not parsed correctly. Now they should be fine.

Before:

```
htscmd vcfquery -f '%CHROM\t%POS\t%REF\t%ALT\t%INFO/GENE_NAME\n' file.vcf.gz
Error: no such tag defined in the VCF header: INFO/GENE
```

After:

```
htscmd vcfquery -f '%CHROM\t%POS\t%REF\t%ALT\t%INFO/GENE_NAME\n' file.vcf.gz
Error: no such tag defined in the VCF header: INFO/GENE_NAME
```