Update validation script command line interface

[andrewhill157]

I would like to make this similar to the generate_annotated_vcf.py interface, where you can provide a list of files rather than a file containing a list of names as input. This will
allow the following:

cat file_names.txt | validate_annotated_vcfs.py -o output.vcf -d discordant.vcf
# OR 
validate_annotated_vcfs.py -o output.vcf -d discordant.vcf file1.vcf file2.vcf -f