Added JSON file to specify chromosome --> VCF file mappings

Instead of providing a single merged VCF, you can give --ld-vcf a JSON
file mapping chromosome to VCF.
Fix for EPACTS changing file format

EPACTS public and EPACTS test have different formats now, both header
and column order have changed. LZ now has options to denote the names of
the columns. The defaults are:

--epacts-chrom-col #CHROM
--epacts-beg-col BEGIN
--epacts-end-col END
--epacts-pval-col PVALUE

In TEST, EPACTS calls BEGIN --> BEG, and PVALUE is no longer the 8th
column.