custom liftover maps

When Ensembl doesn't provide a reasonable reference, or doesn't support a particular assembly, it's useful to be able to use a custom liftover map provided e.g. as a chain file: https://genome.ucsc.edu/goldenPath/help/chain.html

1. import/specify liftover map
2. Allow easy specification of which map to use when importing - e.g. VCF files.