Make pointer to snpsift

[cluebbert]

Right now panvaR assumes programs are in the user's /bin directory. If they aren't there's not a good way right now to change this.

Like we did with set_plink_path, we should do the same with snpsift.

[cluebbert]

I think it might be good to remove this bit from the program itself. Right now it is a requirement to have this but I think overall I would like to move to a framework where the program does a base calculation of LD then the user can supply any number of snp specific variant effect scores.

In this framework, we could provide documentation of how to extract snpeff info from a vcf or have optional functions that can do this but I don't think it should be mandatory to run the program as it is now.