add -single file output option to ADAM2Vcf

[jpdna]

Add a -single option to ADAM2Vcf to output a single VCF file.
I assume this can be done similarly to how it is done currently for BAM output.

@fnothaft - let us know what you think the best approach is here. I am willing to work on porting over to VCF, but I know this was a finicky bit of code, so if you'd rather do it or if you have suggestions let me know.

My near term goal here is to examine the VCF transformations/roundtrips using each of Parquet and HBase data stores, both of which can make use of this same code for VCF output.

[fnothaft]

I'm going to close this as a dupe of #1009. Thanks for opening it; it gave me the impetus to get back to fixing this. I should have a PR tomorrow.