Enable "variants-only" mode for VariantContext2VariantConverter

[laserson]

Currently, VariantContext2VariantConverter will load all genotype calls from a VCF file into the resulting GA4GH Variant object. Sometimes the user may only want the variant data itself, without any of the associated calls. This should be a configurable option

[imedina]

I agree, I think this was discussed some time ago. Actually, other modes can be added, one proposal could be:

• skip-non-variants: current gVCFs contain both variants and non-variants calls, sometimes it can be useful to load only real variants and skip non-variants calls
• exclude-genotype: this is the one you meant above with 'variants-only'