add exac frequency support
add exac frequency support

willmclaren/ensembl-variation,willmclaren/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation