Add support for GWAS data in Plink format

[mnwright]

This should be as fast as using the GenABEL interface but with Plink BED/BIM/FAM files.

Example:

ranger(y ~., pheno, plink.file.prefix = "path_to_plink_data")

or

ranger(y = y, x = x, plink.file.prefix = "path_to_plink_data")

or without covariates except SNPs

ranger(y = y, plink.file.prefix = "path_to_plink_data")

Prediction works similarly:

predict(rf, pheno, plink.file.prefix = "path_to_plink_data")

or

predict(rf, plink.file.prefix = "path_to_plink_data")

Please try it out and report any issues!